Diane M. Purdy, 48, a retired teacher in the Prince William County school system who was active in civic groups, died of cancer Monday at Georgetown University Hospital in Washington.

Mrs. Purdy was a native of Oakland, Calif. She was a 1954 graduate of the University of California at Berkeley, where she majored in psychology. In 1958, she married Harry E. Purdy, an Army officer who is now a retired lieutenant colonel.

Mrs. Purdy moved to the Washington area in 1968 and settled at Lake Montclair in Prince William County. Until 1980, when she retired, she taught the third grade at the Neabsco Elementary School in Dale City and the Rippon Elementary School in Woodbridge.

She was a founder and a member of the Lake Montclair Garden Club and was a docent at Gunston Hall Planation, the home of George Mason, in Lorton. She also was a member of the parish of St. Margaret's Episcopal Church in Woodbridge.

In addition to her husband, of Lake Montclair, Mrs. Purdyhs survivors include four daughters, Jewel, Deirdre, Laura and Dana Purdy, all of Lake Montclair, and her mother, May Wishart of Oakland, Calif.

The family suggests that expressions of sympathy be in the form of contributions to the Vincent T. Lombardi Cancer Research Center, Georgetown University Hospital, 3000 Reservior Rd., Washington, 20007.