Annual variation of the surface mixed layer depth (MLD) in the southern part of the Sea of Japan is investigated based upon the oceanographic and meteorological data taken during 1971∼1975 by the Fisheries Research and Development Agency and the Central Meteorological Office of Korea. It is found that the variation of the MLD is strongly correlated with the heat exchange between the atmosphere and sea. The MLD and heat flux vary within ranges comparable to those in the Kuroshio region found by Bathen(1972) and Wyrtki(1965)