This article discusses the horizontal distribution and dominant species of phytoplankton in the East China Sea. The ecological characteristics of phytoplankton and their adaptabilty to the environment were also considered. Oceanographic investigation was carried out in the East China Sea (23°30′-33°N and 118°30′-128°E) in four seasons from 1997 to 2000. It was found that the total abundance showed obvious seasonal variations. It peaked in autumn with a mean value of 211.91×104 cell•m-3, followed by summer (50.40×104 cell•m-3). The lowest abundance occurred in spring (2.01×104 cell•m-3). 4 seasons excluding winter, the mean density of phytoplanktonwas over 100×104 cell•m-3. For the horizontal distribution, abundance in summer and spring was higher in nearshore than in offshore areas of the East China Sea. In autumn and winter, abundance in the offshore of the north was higher than in the nearshore, while in the south this was reversed. The aggregation characteristics of phytoplankton were obvious from multivariate regression analysis. Over the 4 seasons, phytoplankton abundance showed a significant correlation with the water temperature. Eleven dominant species were observed in 4 investigated seasons, in which Chaetoceros lorenzianus and Thalassiosira subtilis dominated in winter, Chaetoceros lorenzianus and Noctiluca scientillans in spring, Chaetoceros pseudocurvisetus and Rhizosolenia alataf. gracillima in summer and only Chaetoceros socialis in autumn. Temperature was found to be a major influencing factor whereas salinity was a minor one. Comparing their adaptability, Chaetoceros lorenzianus and Biddulphia sinensis can survive in a wider temperature range (12-28℃), which enables them to dominate in spring, summer and winter, while Skeletonema costatum survive in a relatively narrow temperature range (22-28℃) and the temperature range for Chaetoceros socialis was even more narrow (21-25℃). The indicator species in the East China Sea can be categorized into two different groups, according to their abundance and aggregation characteristics. One is marked by the occurrence of the species, and the other is marked by the presence of their high abundance area. Chaetoceros lorenzianus、Stephanopyxis palmeriana、Rhizosolenia robusta、Rhizosolenia calcar-avis contribute to the offshore warm current species.