To understand the phytoplankton community in adjacent waters of Ulchin nuclear power plant (UNPP), abundance and the size fractionated $chl-alpha$ concentrations were evaluated through seasonal interval sampling from April 2003 to February 2004. A total of 211 different phytoplankton species was observed and mean abundance of phytoplankton in each study period ranged from 244,286 to 1,221,779 cells $L^{-1}$. The contributions of microplankton $(>20mu{m})$ to total phytoplankton abundance ranged from 42.5 to 83.6% (average 66.1%) and those of nanoplankton $(>20mu{m})$ ranged from 16.4 to 57.5% (average 33.9%). Total chl-$alpha$ concentrations of phytoplankton ranged from 0.52 to $2.26mu{g};L^{-1}$. The contribution of chl-$alpha$ concentrations of microplankton was higher than that of nano- and picoplankton through the study period with exception of July 2008. The results of abundances and $chl-alpha$ concentrations suggest that microplankton has an important role in adjacent waters of UNPP. The diminution of abundances and $chl-alpha$ concentrations of phytoplankton was observed after passage through the cooling water system, but it was gradually recovered by mixing with the ambient waters. Our results suggested that the influence of thermal discharges on phytoplankton should be restricted within narrow limits around outlet area of thermal effluents.