In order to investigate the distribution of phytoplankton community in the coastal waters of the Chuksan Harbor, East Sea, the abundance and biomass of phytoplankton have been evaluated through seasonal interval sampling from April 2000 to October 2002. A total of 363 different phytoplankton species was observed and most of them were composed of diatoms. The mean abundance and chlorophyll-a concentration of phytoplankton during the study period ranged from 56 ${ imes}$ $10^3$ to 720 ${ imes}$ $10^3$ cells $L^{-1}$ and from 0.78 to 3.29 μg chl-a $L^{-1}$, respectively. The relative contribution of the size-fractionated phytoplankton to phytoplankton community showed difference according to seasons. The average contribution of nano-phytoplankton(<20 $mu$m) was over 50% in the total abundance and biomass of the phytoplankton. Our results show that nano-phytoplankton play an important role in the southern coastal waters of the East Sea. And the environmental factors such as suspended substances, phosphates and silicates were positively correlated with the abundances and biomass of phytoplankton.