Phytoplankton and zooplankton communities were investigated from 8 weir sites of the Nakdong River system to provide basic information on plankton community after weir construction and to estimate its effects on major water quality parameters. The relationship between plankton community structure and environmental factors was analysed with CCA (Canonical Correspondence Analysis). The results suggested that discharge and total phosphorus and nitrogen concentrations are important factors determining the phytoplankton species composition. For zooplankton community, the difference in discharge between September and October induced different distribution pattern of zooplankton community with more homogeneous distribution with extreme dominance of rotifers during the high discharge season. Chlorophyll a concentration representing phytoplankton biomass has been suggested as the main environmental factor affecting zooplankton community followed by COD and total nitrogen concentration.