The seasonal changes and composition of phytoplankton of the Choonsan reservior were studied from November 1990 to October 1991. 195 taxa which belong to 65 genera, 150 species, 40 varieties and 1 forms were identified. Species number during summer monthes was higher than that of other seasons. Standing crops of phytoplankton were varied from $7.8 imes10^5$ cells/l to $2.2 imesl0^6$ cells/$ell$. The dominant species were Cryptomonas erose and Trachelomonas volvoina. Chlorophyll a contents of phytoplankton was varied from $1.36mu extrm{g}/ell$ to $127.46mu extrm{g}/ell$. The biotic indices of phytoplankton were very similar among three sites. Saprobien-system by index of species diversity suggests that the Choonsan reservior belongs to the oligosaprophic.