With physico-chemical environmental factors such as temperature, DO, pH, conductivity and nutrients, size fractionated of phytoplankton standing crops and chlorophyll a concentrations $(>8mu{m},;3-8mu{m},;<3mu{m})$ and their relationships were investigated at each station of Kyungan Stream and Togyo Reservoir from April to November in 1997. The two sampling sites showed different nutrient status: Kyungan Stream was eutrophic, while Togyo Reservoir was mesotrophic. Large sizes of phytoplankton and chl. a were higher in Kyungan Stream, opposite to those of Togyo Reservoir; Standing crops of phytoplankton $(>8mu{m)$ and chl. a $(3-8mu{m)}$ were high in Kyung-an Stream, while phytoplankton $(3-8mu{m)}$and chi. a $(<3mu{m)$ were abundant in Togyo Reservoir. These results imply that phytoplankton community in the highly eutrophicated water mainly comprised the large filamentous and/or colonial algae, such as Microcystis spp. and Aphanizomenon flos-aquae, which easily enriched by nutrients loading.