Gerald Cullinan, 63, who was assistant to the president of the National Association of Letter Carriers, died of cardiac arrest Thursday at Sibley Memorial Hospital.

He had served with the letter carriers from 1960 until retiring earlier this year.

He had been a confidential assistant to postmaster general Arthur E. Summerfield and then special assistant to the deputy postmaster general during the Eisenhower administration.

Mr. Cullinan was born in San Francisco and graduated from St. Ignatius High School there. He then attended Oxford University in England where he earned a bachelor's degree in 1937 and a master's degree a year later.

Before World War II, Mr. Cullinan was a reporter for the San Francisco Call Bulletin. He served with the Army Air Forces in World War II, and was a partner in a public relations firm in Dallas after the war. He came to Washington in 1953.

Mr. Cullinan had been a political speech writer for several members of Congress. He wrote several books, including "The Post Office Department" and "The United States Postal Service."

He is survived by his wife, the former Barbara Lynch, of the home in Washington; two children, Thomas A., of New Orleans, and Mary Patricia, of Berkeley, Calif., and a brother, Vincent, of San Francisco.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society.