Cyrus S. Eaton, 95, the multimillionaire who sought to improve relations between the United States and the Soviet Union and other communist states, died Wednesday night at his estate at Northfield, Ohio, a suburb of Cleveland.

The announcement of Mr. Eaton's death was delayed so that his son, Cyrus S. Jr., who was traveling in China, could be notified. The cause of death was not disclosed.

Mr. Eaton began his career as a messenger boy for John D. Rockefeller Sr., who founded the Standard Oil empire and who acted as a surrogate father to the young Eaton. Mr. Eaton himself became a magnate in the classic capitalist mold with extensive holdings in steel, rubber, railroads, mining, shipping and banking. His fortune has been estimated at nearly $2 billion.

He knew every president since Theodore Roosevelt and his advice was sought by such diverse chief executives as Herbert Hoover and Harry S. Truman.

"I am an incurable and dedicated capitalist," he said in a retrospective interview in 1973.

He pursued his vision of peace and detents among the big powers with the same vigor he brought to his business enterprises. He was a friend of Nikita S. Khurshchev and Alexei N. Kosygin, Khrushchev's successor as premier of the Soviet Union. Mr. Eaton visited North Vietnam in 1969 and met with Premier Pham Van Dong there. He was an outspoken proponent of U.S. withdrawal from Vietnam.

In 1955, he established the Pugwash Conferences, which since have become annual meetings between scientists, scholars and others interested in lessening tensions in the world and reducing the threat of nuclear annihilation. The conferences are named after Mr. Eaton's birthplace in Nova Scotia, Canada, where the first of them was held. In 1960, he became one of the few Americans ever to receive the Lenin Peace Prize.

In an interview with The Washington Post in 1975, Mr. Eaton said he planned to "devote all of my waking hours from now on to persuading the Americans and the Soviets to give up their ideas about military supremacy. We develop these instruments of destruction to slaughter the children of the world. It's a hideous conception."

Mr. Eaton became interested in Russia as a young man through his friendship with Prof. Samuel N. Harper of the University of Chicago, one of the first American academic experts on Imperial Russia and the Soviet Union. But his first visit to Moscow did not come until 1955.

The Cold War was at its height; but Mr. Eaton Nonetheless advocated trade between the United States and the Soviet Union. In 1959, he entertained Soviet Deputy Premier Anastas I. Mikoyan at Cleveland's ultraconservative Union Club. Executives of leading steel and rubber companies were among the guests.

"Mikoyan wanted to buy and they were eager to sell," Mr. Eaton recalled later. "But Washington said no."

The man whom Forbes, the business publication, once identified as one of the 20 wealthiest men in America was interested in things of the mind as weel as those of the marketplace. He was a member of the American Council of Learned Societies, the American Historical Association and the American Philosophical Association. He was a trustee of the University of Chicago.

An avid and electric reader, Mr. Eaton delved into history, science, and philosophy, but more in the classic vein than in the modern Charles Darwin and Thomas Huxley-whose sons he counted among his friends-and Herbert Spencer were among his "newer" favorites.

"You know," he said in the 1973 interview, "I've never seen a movie. Not that I have anything against them, but I just haven't found the time."

Mr. Eaton was a devotee of vigorous sports, playing ice hockey until he was 70, tennis until he was 75, skiing until he was 80, and riding horseback into his 90s. He walked at least an hour each day, mostly on his 860-acre estate near Cleveland or at one of his two farms in Nova Scotia.

He bred and raised Scotch Shorthorn cattle and with them won many international prizes. He also raised horses, thoroughbred show animals at first, then quarter horses, and latterly colts bred from quarter-horse stock and three Russian Orlov stallions given to him by Khruschev.

Cyrus Stephen Eaton was born on Dec. 27, 1883. His father, Joseph H. Eaton, ran a general store in Pugwash and had three farms. His mother was Mary McPherson Eaton. Both families were of Scottish stock.

Young Cyrus immigrated to the United States when he was 17 to live with an uncle, the Rev. Dr. Charles Eaton, Baptist minister who in later life became a member of Congress from New Jersey and chairman of the House Foreign Affairs Committee. In 1913, Mr. Eaton became a U.S. citizen.

Dr. Eaton's Euclid Avenue Baptist Church happened to be John D. Rockefeller's place of worship. Also, his home outside Cleveland adjoined Rockefeller's estate, "Forest Hills," where the multimillionaire stayed from May through September of each year. The period coincided roughtly with young Eaton's vacations from McMaster University at Hamilton, Ont., and he went to work for the man he later called "the greatest business genius in history."

After graduation, Mr. Eaton preached a few sermons in Baptist churches, although never officially a minister, and then turned to business. By 1907, when he was 24, he was representing Rockefeller interests in Canada that were trying to put together a public utilities organization. But there was a financial panic at the time and a New York syndicate supporting the project backed out. Its head suggested that Eaton go to Canada, which was not affected by the American financial distress, and raise the money for himself.

"In Montreal," he said in 1973, "I ran into Max Aiken, later Lord Beaverbrook. He was born near me in Nova Scotia. He helped me get the money together and I started operations in Saskatchewan and Manitoba."

From this beginning Mr. Eaton built a public utility empire that by the middle 1920s was the third largest in the nations. In 1925, he bought a financially troubled steel company in Warren, Ohio, with a personal check for $18 million, added other companies to it and formed the Republic Steel Corp. Eventually, Republic became the third largest steel producer in the United States.

Attempting to expand his steel operations, Mr. Eaton ran into opposition from Bethelem Steel, the nation's second largest after U.S. Steel, and its redoubtable operators, Charles M. Schwab and Eugene Grace. A long and expensive court suit over control of Youngstown Sheet & Tube ensued. Mr. Eaton won, just before the 1929 Wall Street crash and the ensuing Depressiion.

Mr. Eaton was virtually wiped out. But he retained large holdings in Otis & Co., a Cleveland investment banking firm. Although it, too, was hard hit by the Depression, Otis formed the basis with another Eaton company, Cleveland-Cliffs Iron Co., as the foundation for a new fortune.

During the 1930s, Mr. Eaton picked up stocks in undervalued companies, including many steel companies. By the 1950s, through these companies, he controlled more than 1 million shares in six major procedures. He also bought heavily in the Chesapeake & Ohio Railroad, so much that when Robert Young left that company to take over the New York Central in 1954, Mr. Eaton became chairman of the board. He remained chairman of the Chessie System until 1973 and a board member until 1978.

Coal also was among Mr. Eaton's post-world War II ventures. (The C&O was the world's largest bituminous coal hauler). He bought control of West Kentucky Coal in 1953, then expanded other coal areas in the Appalachians.

Other corporate interests included Detroit Steel Corp., Cleveland Electric Illuminating Co., Kansas City Power and Light and Sherwin-Williams co.

At one time he was the controlling figure in Goodyear Tire & Rubber and was heavily involved in both Firestone and B. F. Goodrich.

Although the image of the quintessential capitalist, Mr. Eaton paradoxically was an enthusiast for Roosevelt's New Deal and a friend of president Truman, whom he came to know when Truman was a junior senator. The core of this unusual attitude, perhaps, stemmed from his lifelong opposition to Wall Street influence.

"The U.S.," he once said, "owes its unparalleled economic progress to its creative geniuses and not its moneylenders. $

Mr. Eaton's first direct contact with Russians came in 1955, when the State Department's exchange program had a touring group of Soviet journalists on hand who wanted to meet a "capitalists." A member of the group was Khrushchev's son-in-law, Alexel Adzhubel.

Mr. Eaton entertained the journalists in Cleveland, and became a "person-to-see" for succeeding Soviet vistors. However, Soviet and American security officials balked at allowing Khhrushchev to go to Cleveland, with its many Slavic residents, when the Communist leader visited the United States in 1959. So Mr. Eaton held a big luncheon for him at New York' Waldorf-Astoria Hotel.

During this period, Mr. Eaton became vocal about the need for improving East-West relations and was labeled by some as a "fellow travelers" who was "soft on communism." He refused to renounce his views.

During the Vietnamese war, besides making a visit to Hanoi, Mr. Eaton went to the Soviet Union, and had lengthy talks in May 1965, with Kosygin and Mikoyan. He came back, warning that the Soviet Union would join with the Communist China to attack the United States if bombing of North Vietnam was not halted.

He said in the 1973 interview that at that time he urged president Lyndon B. Johnson to listen to the Soviet warnings and to improve relations with the Russians.

Of the tentative Soviet-American detente reached by the Nixon adminstration, Mr. Eaton was cautiously hopeful. "We just have to develop trade with them," he said. "And not only with the Soviet Union, but China as well. We can't put religious or political obstacles in the way."

In 1974, Mr. Eaton visited Fidel Castro in Havana and said later that the Cuban leader wanted to restore relations with the United States.

He lived to see the United States and China establish full diplomatic relations and to see a tentative new agreement between the United States and the Soviet Union limiting strategic arms.

Mr. Eaton had seven children by his first wife, the late Margaret House Eaton, of whom six survive. In 1957, 20 years after his first wife's death, he married Anne Kindner Jones, who, by chance, had been a classmate of his youngest daughter in boarding school.

The second Mrs. Eaton who taught herself Russian so she could help her husband in his conversations with Soviet leaders, also survives, as does one stepdaughter. CAPTION: Picture, Mr. Eaton, photographed in 1975 during an interview with a Post reporter By Larry Morris-The Washington Post