Robert B. Stewart, 70, a former service station owner and a retired teacher of military science at Cardozo High School, died of cancer Thursday at Walter Reed Army Medical Center. He lived at Rossmoor Leisure World in Silver Spring.

Mr. Stewart, who was born in Lynchburg, Va., and grew up there, came to Washington in 1929 as a student at Howard University, from which he graduated in 1933. From 1935 to 1942, he was a member of the D.C. police department.

During World War II, he served in the Army at Fort Dix, N.J., and at Fort Hood, Tex., where he commanded a tank destroyer unit. He was retired on disability with the rank of captain because of injuries received in a training accident.

After leaving the service, Mr. Stewart was a cab driver in Washington. From 1956 to 1959, he was a coowner of the Chapel Oaks Esso Service Station in Chapel Oaks, Md. From 1960 until his retirement in 1966, he taught military science and tactics at Cardozo High.

Mr. Stewart's hobbies included fishing and golf and he was a member of the Rossmoor Golf Association.

His first wife, the former Carlyn Wilson, died in 1968.

Survivors include his second wife, Eloise L. Stewart of Rossmoor; two daughters by his first marriage, Patricia S. Hatcher of Washington and Lisa L. Stewart of Arlington; one brother, Colston R., of Washington, and two grandchildren.