On the basis of seasonal investigations at 23°30′～33°00′N, 118°30′～128°00′E of the East China Sea during 1997～2000, dynamics on the density and diversity of Ostracoda was discussed. Results showed that totally 26 species were identified. The Ostracoda diversity was opposite to the change of its density in most seasons which reflected an uneven assignment of Ostracoda density among its different species. The Ostracoda density was 0.70 ind./m3 in spring, 1.72 ind./m3 in summer, 2.57 ind./m3 in autumn and 0.90 ind./m3 in winter. Euconchoecia chierchiae in spring and winter, Euconchoecia maimai in summer and Cypridina dentata in autumn were main dominant species in each season. The Ostracoda density did not show an obvious linear relationship with the hydrologic factors in summer and autumn, but was related to the surface salinity in spring and the surface temperature in winter. Its high density areas mainly distributed in the north offshore in all the seasons while in the south offshore in winter and in spring, and the south nearshore in summer and autumn, implied the zooplankton was a typical warm water animal, whose high density distribution in autumn were located in a similar position to Todarodes pacificus, Navodon Septentrionalis, Scomber japonicus and other fishes in the sea, so as to be an important indicator for fishing ground. The main species dominating in Ostracoda now are different from the species twenty years ago probably attributes to global warming.