Harley O. Preston, 66, retired director of external affairs of the American Psychological Association, died Thursday at Arlington Hospital after a long illness.

He had joined the association here in 1964 and retired in 1975. Before coming here, he was director of international services of the American Institutes for Research in Pittsburgh. He had served with AIR since 1947.

Born in Lakewood, Ohio, Dr. Preston was a graduate of Kent State University and held master's and doctoral degrees in sociology from Indiana University.

He was a research and teaching assistant at Indiana University from 1938 to 1941. Then he became an instructor and assistant professor at Kent State until 1947. From 1942 to 1946, however, he was on leave from Kent State to serve in the aviation psychology program of the Army Air Forces.

After retiring from the American Psychological Association, Dr. Preston was a consultant with the Human Resources Research Office of George Washington University and with the Bureau of Social Science Research.

He was the author of the numerous articles on military psychology and international affair for professionals Journals.

He belonged to the American Sociological Association, the International Council of Psychologists, the Society for Applied Anthropology, the American Management Association and the National Planning Association.

Dr. Preston also was a member of the American Society for Public Administration, the Inter-American Society of Psychology, the Virginia Psychological Association and the Society for International Development.

He is survived by his wife, Alice Strey Preston, of the home in Arlington; a daughter, Martha Flukinger, of Tex., and two sisters, Jessie Swearingen and Frances Hutchison, both of San Leandro, Calif.