Donald R. Thomas 55, a professor of education at American University and a former dean of its school of education, died of cancer Tuesday at George Washington University Hospital.

Dr. Thomas joined the AU faculty in 1972. In addition to teaching, he was dean of the school of education from 1972 to 1976, and a chairman of the university Senate.

His particular interest was education for disadvantaged children. In the course of his career, in California, Wisconsin and Ohio, he directed several projects on the education of the children of migratory workers and deprived children.

Dr. Thomas was born in Oakland, Calif. He served in the Marine Corps during World War II, and then earned a bachelor's degree in education at the University of california at Berkeley.

He later taught in public schools in California and then spent two years teaching in Cali, Colombia. In 1953 he returned to California and was a demonstration teacher at San Francisco State College from 1954 to 1957. At the same time, he earned master's and doctoral degrees in education at Stanford University.

Dr. Thomas was on the faculty of the University of Wisconsin from 1957 to 1960, and the faculty of California State University at Northridge from 1961 to 1967. He spent the next five years at Case Western Reserve in Cleveland, and then came to American University.

He was a member of the American Association of University Professors and the American Association of Colleges of Teacher Education.

Survivors include his wife, Sarah, of the home in Rockville; three sons, Jordan, of Laurel, Lanny, of Los Angeles and David, of the home; two daughters, Melinda, of El Cerrito, Calif., and Julie, of the home; his mother, Lillian Thomas, and a brother, Phil, both of Oakland, another brother, Hugh, of Los Angeles, and one grandchild.

The family suggests that expressions of sympathy be in the form of contributions to the American Cancer Society.