The distribution and standing crops of phytoplankton were ivnestigated seasonally at 5 stations in Juam lake in 1993. It was identified as total 175 taxa, and consists of 31 families, 59 genera, 151 species, 19 varieties, 4 forma and I unidentified species according to Engler`s classifi-cation system. The dominant species were Microcystis aeruginosa, Staurastrum longi-radiatum, Eudorina unicocca, Fragilaria crotonensis, Melosira ambigua and Cyclotella stelligera. The standing crops was the highest value in summer because of the appro-priate condition for the photosynthesis in irradiation and temperature. It was estimated that the nutrient level was changing from oligotrophic to eutrophic state as for the dominant species.