The seasonal changes in phytoplankton species composition, standing crops, dominant species, species diversity and physico-chemical characteristics in the Unmun dam were studied, from August, 2009 to April, 2010. The phytoplanktons of a total of 121 taxa were identified, the composition of phytoplankton community was characterized by a green algae and diatoms, and the quantity composition of standing crops was dominated by diatoms and dinophyceae. The diatoms Aulacoseria granulata and blue-green alga Anabaena planktonica in Summer, Peridinium voltzii in Autumn and Asterionella formosa in Winter to Spring were, especially, prominent. The seasonal changes of the biomass varied from 578 cells $mL^{-1}$ to 12,938 cells $mL^{-1}$, and the maximum algal density was observed in April, Asterionells formosa contributed to 88% of the total cell numbers. The species diversity and richness were highest during autumn, and dominance index was highest in the spring season.