We investigated the characteristics of the thermohaline structure and phytoplankton community from the southwestern areas of Jeju to the nothern areas of Taiwan in the Ease China Sea, in June 2004. According to the analysis of a T-S diagram, three characteristic of water masses were identified. We classified them into the mixed water mass by the Chiness continental coastal waters and Yellow Sea cold water (Region A), Chinese continental coastal waters (Region B) and Taiwan warm current (Region C). Region A was characterized by low temperature, low salinity, high density and high Chl-a concentration. Region B was characterized by high temperature, low salinity, low density and high Chl-a and Region C was characterized high temperature, high salinity, low density and low Chl-a concentration. The phytoplankton community identified a total of 56 species belonging to 31 genera. The dominant species was mainly dinoflagellates, Gymnodinium breve, Scrippsiella trochoidea, Ceratium fusus, Prororcentrum triestinum, centric diatoms, Chaetoceros lorenzianus, Leptocylindrus danicus, Proboscia alata, Skeletonema costatum and pennate diatoms, Pseudonitzschia pungens, Cylidrotheca closterium. Standing crops of phytoplakton fluctuated between 0.1×102 cells/L and 5.7×104 cells/L by dominance of dinoflagellates. In the phytoplankton community, the Region A was characterized by the various species composition in 39 species, the dominint species with di-atomes, Pn. pungen, Ch. lorenzianus and standing crops from 6.9 cells/ml to 56.6 cells/ml, Region B by the various species composition in 37 species, the dominant species with dinoflagellates, G.breve, S. trochoidea and standing crops from 4.6 cells/ml to 26.7 cells/ml, and the Region C by low species number with 28 species, the dominant species with one dinoflagellate, S.trochoidea and one diatom, L.danicus and very low standing crops from 0.1 cells/ml to 5.7 cells/ml. Phytoplankton productivity in the East China Sea was controlled by Chinese continental coastal waters which include a high concentrations of nutrients.