The physical factors and chemical contents were studied at 8 stations within Masan Bay, differing in depth and location with respect to inner and outer harbours. It is a relatively small bay with a long and narrow outlet. The water temperature and salinity at the inner bay was rather subject to change on weather condition of the neighbouring land than on the effects of the water mass of outer bay. The high nutrient contents in the bay were caused by the sewage and industrial activities. The outer harbour was less polluted than that of the inner bay; the high contents were significantly decreased from the entrance of the outer harbour. While the high disolved oxygen contents, over saturation in the surface strata may be the result of nearly all year round phytoplankton blooms, the lack of oxygen contents in the bottom strata were caused by the inadequate mixing of water mass and organic matters. The frequent red tide in the area may be the results of inflowing raw sewage, industrial activities of neighbouring land and inadequate mixing of water masses.