Abstract - The seasonality of phytoplankton in Dongbok lake was analysed from March to November 2003. The concentrations of TN and TP showed nearly constant level except high concentrations in May at dam site of Dongbok lake. Chlorophyll a concentration was highest at dam site in May with 225.3 μg L-1, and high in spring and fall and low in summer at upper and central regions of Dongbok lake. A total of 108 phytoplankton species was identified as an algal flora of Dongbok lake. They were 54 Chlorophyceae, 30 Bacillariophyceae, 12 Cyanophyceae, and 12 species of other taxa. Total cell biomass of phytoplankton showed peaks in May~June and August ~September, and low biomass in July at dam site. However, upper and central regions of Dongbok lake showed no clear patterns in cell biomass. Maximum biomass was 7,158 cellsmL-1 at dam site in May with the blooms of Peridinium bipes f. occulatum. The general seasonality of phytoplankton in Dongbok lake was Bacillariophyceae-Dinophyceae/Bacillariophyceae-Cyanophyceae/ Chlorophyceae/ Bacillariophyceae-Bacillariophyceae in 2003. Key words : phytoplankton, chlorophyll a, seasonality, Dongbok Lake