William Joseph Hassan, 68, who served as commonwealth's attorney of Arlington County for 24 years before his retirement in 1974, died Monday at the Veterans Administration Hospital in Wilmington, Del. He had suffered a stroke and had diabetes.

Mr. Hassan was elected commonwealth's attorney in 1950, defeating incumbent Denman Rucker Sr. During his years as the county's chief prosecutor, he obtained the murder conviction of John C. Patler for the slaying in 1967 of George Lincoln Rockwell, the American Nazi leader. Mr Hassan regared this as one of the high points of his career.

He retired in February 1974 for reasons of health.

Mr. Hassan was born in Mattapan, Mass. He attended Walbash College and later worked for the Works Progress Administration during the Great Depression.

In 1935, he was appointed a Secret Service agent was assigned to the detail guarding President Franklin D. Roosevelt and his family. During World War II, he served as an intelligence officier in the Coast Guard, in which he rose to the rank of lieutenant commander. He returned to the Secret Service after the war and earned his law degree at Catholic University by attending night classes. Earlier, he had attended Harvard Law School.

He was elected Arlington commonwealth's attorney on his first try for public office and was reelected six subsequent times.

Mr. Hassan was a member of the Federal, Arlington and Virginia bar associations. He was a past president of the Arlington Democratic Club, a past president of the Freindly Sons of St. Parick and a past president of the Virginia Commonwealth Attorneys Association. He also was a past deputy of the Knights of Columbus of Massachusetts. He received the Silver Beaver award from the Boy Scouts of America in early 1960s.

In addition, Mr. Hassan was a member of the American Legion and the Veterans of Foreign Wars.

Survivors include his wife, Lillian M., of the home in Stafford, Va.; a daughter, Carole M. Burns of King George, Va.; three sons, William E., of Fairfax, Patrick D., of Fredericksburg, Va., and Michael S., of Arlington; a brother Denis H., of Plymouth, Mass., and five grandchildren. a