By use of C. S. K. summer (1977) and winter (1974) data, the distribution of water types in the East China Sea is studied. In summer, it is classified into the Yellow Sea Cold Water, the Coastal Water, the East China Sea Water, the Mixed Water, the Tsushima Warm Water, and the Kuroshio Water. Seasonal thermocline and halocline appear strongly in the surface layer of the Yellow Sea Cold Water and the Coastal Water, and the maximum value of dissolved oxygen exists at the depth at which seasonal thermocline shows up. The Coastal Water which is formed by mixing with the outflow fresh water of the Yangtze River distributes itself at the surface Iayer because of the lower density. On the other hand, in winter, the Coastal Water does not appear and the characteristics of the Yellow Sea Cold Water and the East China Sea Water are homogeneous vertically. This is because the surface water is mixed vertically with the influence cooling. In both seasons, the Mixed Water appears in the marginal region of the continental shelf and in the continental slope. We can see that the Tsushima Warm Water and the Mixed Water are almost plotted on the same region in the temperature-salinity diagram. The Tsushima Warm Water does not originate from the main flow of the Kuroshio, but from the Mixed Water on the marginal region of the continental shelf and the continental slope.