Thomas K. Finletter, 86, former secretary of the Air Force and later U.S. ambassador to the North Atlantic Treaty Organization, died Thursday in a New York hospital after a heart attack. He lived in New York City.

Appointed by President Truman in 1950, Mr. Finletter was the second secreatary to head the Air Force Department after its establishment in 1947. sHe succeeded Stuart Symington and held the job until 1953.

President Kennedy appointed Mr. Finletter to the NATO post in March 1961 and he remained in that position until rsigning in July 1965.

When not in government service, he was an active partner in the New York law firm of Coudert Brothers for many years. He remained with the firm until 1971.

A prolific author of books on foreign policy, military power and the atomic and hydrogen ages, Mr. Finletter was working on another book on the dangers of nuclear war at the time of his death.

He long had been a proponent of strong military power for this country to deter potential enemies. After development of the atomic and hydrogen bombs, he also pressed for continuing disarmament talks.

Mr. Finletter's first government appointment came during World War II. From 1941 to 1944, he was special assistant to Secretary of State Cordell Hull. His job was to build up a stockpile of strategic materials, such as tin and rubber, from abroad.

In 1945, he was a member of the U.S. delegation to the United Nations organizing convention in San Francisco.

In 1947-48, he was chairman of President Truman's Air Policy Commission, which recommended a major increase in this country's air power in its report, "Survival in the Air Age."

While he was secretary of the Air Force, Mr. Finletter warned repeatedly that the best insurance against war with Russia was a powerful strategic bombing fleet. After he left his position, he heavily criticized the new Republican administration for cutting defense spending.

A somber man, with a keen analytical mind, Mr. Finletter wrote one of his better-known books, "Power and Policy: United States Foreign Policy and Military Policy in the Hydrogen Age" in 1954. He stated then that Russia could gain atomic-hydrogen air superiority over this country by 1956 and proposed that the United States advocate a complete compulsory disarmament scheme under United Nations control.

Although he was considered to be nonpartisan during his tenure as secretary of Air Force, Mr. Finletter became an active Democrat, serving as chairman of the New York State Stevenson for President Committee in 1956 and then as Adlai E. Stvenson's political advisor.

His return to government service came with his appointment to NATO. When he resigned in 1965, President Johnson praised him for working "tirelessly to strengthen NATO."

Mr. Finletter had been named to another overseas assignment earlier in his career. In 1948-49, he was minister in charge of this country's Economic Cooperation Administration's Mission (Marshall Plan) to the United Kingdom.

He was born in Philadelphia. He graduated from the University of Pennsylvania, earning a law degree there after serving as a U.S. Army field artillery officer in France in World War I. He became a partner of Coudert Brothers in 1926.

Mr. Finletter had been a member of the Council on Foreign Relations. His clubs included the Metropolitan Club in Washington.

His first wife, the former Gretchen Damrosch, whom he married in 1920, died in 1967. He married the former Eileen Wechsler in 1972.

In addition to his wife, survivors include two daughters, Margo F.Mitchell of Stonington, Conn., and Lili O'Neill of Los Angeles, and two grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the United Nations International Children's Emergency Fund (UNICEF).