Thomas S. Gates Jr., 76, a secretary of defense in the Eisenhower administration and a chief of the U.S. liaison mission to Peking in the Ford administration, died yesterday at Pennsylvania Hospital in Philadelphia. The cause of death was not reported.

Mr. Gates was a banker by profession and a former chairman and chief executive officer of the Morgan Guaranty Trust Co., the fifth largest in the nation. He was a Philadelphia Main Liner by birth and a Navy veteran of World War II.

As a public servant, he earned a notable reputation as a manager, organizer and strategist during the seven years in which he held high office in the defense establishment. During his tenure as secretary of defense, which extended from 1959 to the end of the Eisenhower administration on Jan. 20, 1961, he instituted some of the managerial techniques for which his successor, Robert S. McNamara, became famous.

Mr. Gates came to Washington in 1953 as undersecretary of the Navy. In 1957, he was named secretary of the Navy. He resigned on Feb. 3, 1959, planning to return to private life. But at the request of Eisenhower, he stayed on as deputy secretary of defense in place of Donald A. Quarles, who was scheduled to succeed Neil McElroy as secretary. When Mr. Quarles died, Mr. Gates became McElroy's successor.

Asked why he had changed his mind, Mr. Gates said, "I couldn't think of any reason not to do it, except that I didn't want to, and that wasn't good enough."

In the course of his federal service, which included some of the worst years of the Cold War, Mr. Gates was credited with modernizing the Navy by mothballing obsolete ships and concentrating on nuclear powered submarines and aircraft carriers; drawing attention to the necessity of being able to fight limited as well as global wars, and establishing clear administrative guidelines for the Navy and the Marine Corps and otherwise mitigating interservice rivalries.

He once said that his most important contribution as secretary had been setting up a single committee to select targets of retaliation if the nation had to fight a nuclear war. These targets were allocated for all of the services.

In 1970, Mr. Gates headed a presidential commission that recommended abolition of the draft in favor of all-volunteer armed services.

In 1976, President Ford chose him to succeed George Bush as head of the U.S. liaison mission to Peking with the rank of ambassador. Bush, who is now vice president, returned to Washington to become director of central intelligence.

In announcing the China appointment, Ford recalled that he and Mr. Gates had served together aboard the aircraft carrier Monterey during World War II and that Mr. Gates had done an excellent job during his years at Defense. Others recalled that as early as 1965 Mr. Gates had advocated a diplomatic opening with China. This did not come to pass until president Nixon initiated direct ties in 1970 and 1971.

Thomas Sovereign Gates Jr. was born in Germantown, Pa., on April 10, 1906. He graduated from the University of Pennsylvania in 1928 (his father was the unsalaried president of the university from 1930 to 1944) and went into Drexel & Co., his father's investment banking house. He became a partner of the firm in 1940.

After World War II, in which he rose to the rank of captain and served in both the Atlantic and the Pacific, he returned to banking. When he left government in 1961, he joined the Morgan Guaranty Trust Co. He was chairman and chief executive officer from 1965 to 1968 and a director of the bank until 1975.

Mr. Gates, who lived in Devon, Pa., is survived by his wife, Millicent Anne; three daughters, Mrs. Joseph Ponce of Berwyn, Pa., Mrs. William Norris of Bedford, N.Y., and Mrs. Charles McCoy of Malvern, Pa., and nine grandchildren. A son, Thomas S. III, was killed in a fire at a ski resort in Canada in 1956.