Phytoplankton populations were examined at three sites in Lake Cheongpyeong, South Korea from March 2008 to December 2016, including measurement of phytoplankton communities and their dominant species, abundance and environmental factors. The annual average ranges of water temperature, dissolved oxygen and conductivity were 15.2-18.8℃ , 10.3-12.2 mg/L, 86-140 μS/cm, respectively, with similar values at all studied sites. The highest phytoplankton cell density was observed in spring and fall, and it subsequently decreased rapidly during heavy rainfall. Diatoms were dominant in spring (mainly Stephanodiscus hantzschii, Asterionella formosa) and fall (mainly Aulacoseira granulata), while greenalgae and cyanobacteria had high appearance in early-summer and summer, respectively, indicating that water temperature is the most important factor influencing their growth. Stephanodiscus hantzschii and Asterionella formosa frequently occurred at low water temperature (4.5-15.0℃ and 5.4-21.6℃ , respectively) while Aulacoseira granulata and Anabaena spp. were favored by high water temperature (8.6-28.4 and 14.9-26.2℃ , respectively) and phosphorus. Additionally, Fragilaria crotonensis occurred at low nutrient conditions. Rhodomonas spp. frequently appeared year-round.