The chairman of the medical department at the Harvard-affiliated Massachusetts General Hospital (MGH) plans to step down from the post as soon as a search committee can appoint a successor.

Dr. Alexander Leaf, Jackson Professor of Medicine and recently named Ridley Watts Professor of Preventive Medicine, said yesterday he notified University and hospital official of this decision in April, 1979. He said he decided to leave the chairmanship, which he assumed in 1966, because he wanted to devote more time to teaching and research.

Expert

A joint Harvard-MGH search committee, formed after Leaf made his desire to leave the position known, has reportedly completed interviewing candidates and is near a decision. Its chairman, Dr. Gerald Austen, chief of surgical services at MGH, was unavailable for comment yesterday.

The medical department, with more than 500 doctors, is the largest department at MGH, probably the best-known of Harvard's affiliated medical facilities.

Leaf, 60, will become the new chairman of the Department of Preventive Medicine and Clinical Epidemiology after leaving his current position. That department, along with the Department of Social Medicine and Health Policy, was formed in September when the Department of Preventive and Social Medicine split into two divisions.