Charles A. Miller, 64, a retired assistant chief postal inspector, died of cancer Thursday at Sibley Memorial Hospital.

Mr. Miller, who spent more then 30 years as a postal inspector, was a recognized authority on mail fraud, including schemes involving charitable, religious and medical promotions. He was the author of a manual on mail fraud investigations that is used in the U.S. Postal Service. He also wrote a paper on the subject for the National Association of District Attorneys.

He was born in Johnstown, Pa., and joined the postal service there in 1936 as a mail clerk. In 1942, he entered the inspection service. For many years he was stationed in Rochester, N.Y., and was in charge of mail fraud investigations in Upstate New York.

In 1961, he was transferred to the Washington headquarters of the postal service. In 1964, he was named director of the fraud and mailability investigations division and in 1966 he was named assistant chief of criminal investigations in the service. He held that post until his retirement in 1973.

Mr. Miller was a member of the Belle Haven Country Club. In recent years, he divided his time between his home in Old Town Alexandria, and North Palm Beach, Fla. Last February, he moved to Florida permanently.

Survivors include his wife, Jane D. Miller, of the home in North Palm Beach; two daughters, Diane M. Somerford, of Alexandrea, and Jane M. Gehring, of Wauwatosa, Wis., and a sister, Mary M. Peirrel, of Tulsa.

The family suggests that expressions of sympathy be in the form of contributions to the American Academy of Family Physicians, 1740 W. 92d St., Kansas City, Mo., 64114.