To analyze the structure of phytoplankton community of the Asan Reservoir in Korea, samples were collected 6 times from March to November in 1997. A total of 204 phytoplankton species were identified from the samples of 19 stations. Green algae dominated the phytoplankton community, accounting for 51% of species number, followed by diatoms(29%), cyanobacteria(12%), dinoflagellates(2%), euglenoids(3%) and other flagellates(3%). Standing stocks of phytoplankton were very high in the range of 741-613,066 cells/㎖, with the highest standing stock in July. Water Booms seemed to occur in the Asan Reservoir regardless of seasons, with water bloom-causing species being Micractium pusillum, Stephanodiscus hantzschii, Dictyospharium pulchellum, cryptomonad(>20㎛), Microcystis aeruginosa, Oscillatoria tenuis, Oscillatoria sp., Aphanocapsa sp. Euglena sp., Volvox aureus. In the summer, cyanobacteria dominated algal blooms. Species diversity of phytoplankton community in the reservoir was in the range of 0.13∼3.20, and showed much difference temporally and spatially. The cluster analysis identified two different regions of upstream area and downstream area for the Asan Reservoir.