Beatrice Beck Fahnestock, 80, a writer who was a member of the F Street and Sulgrave clubs, died Wednesday in her home in Washington. She had cancer and pneumonia.

She was the author of a series of articles for the old Diplomat magazine in the 1960s entitled "Those Glorious Years." They described Washington society of the 1920s and 1930s. A book of memoirs, "Look Back With Joy," published in the 1960s, told of her friendship with society figures including the Vanderbilts and the Duke and Duchess of Windsor.

Mrs. Fahnestock was a native of Philadelphia and a graduate of Miss Porter's School in Farmington, Conn. She came to Washington in 1937.

She married Somerville Pinkney Tuck, a career foreign service officer, in 1924. She accompanied him to posts in Geneva and Budapest before their divorce in 1934.

She married Col. Snowden A. Fahnestock two years later. Col. Fahnestock, a World War I Army hero and former banker, died in 1962. c

Mrs. Fahnestock's survivors include two sons, James M. Tuck of Mexico and David S. Tuck of the U.S. Virgin Islands; two daughters, Clare Moorhead of Cambridge, Mass., and Mary Lee Leggett of New York City, and six grandchildren.