After liberation of Korea in 1945, he participated in the founding of the Korean Pediatric Science Society in September. In October of that year, with the recommendation of Myung Joo-wan, he became a professor of pediatrics at Seoul National University College of Medicine which lacked professors. He served as vice-chairman of the Seoul Metropolitan Medical Association, and chairman of the Health Policy Committee of the Namjosun Health Federation, but was arrested twice for joining a leftist group. From January 1946, when the movement against trusteeship became a social controversy, university professors decided not to intervene in political affairs, led by Yoon Il-sun at the time, but Lee Byung-nam became a civilian health expert on the left wing.