This study investigated to the composition of phytoplankton community and its ecological factors in the Imha lake changed from the stream environment to the lake environment as the result by the construction of a multiple-purpose dam from Mar. to Dec. in 1992. Phytoplankton community was total 234 taxa composed of 7 class, 14 orders, 6 suborders, 39 families, 95 genera, 199 species, 25 variaties, 3 forma and 7 unidendifed species. The occurred taxa at each station and season ranged from 25 taxa at station 2 on May to 62 taxa at station 4 on May, which were very the wide variations at each station and season. The changes of standing crops were the maximum as 1.5×10^7unit/ℓat station 1 on Mar. and the minimum as 2.9×10^5unit/ℓat station 3 on Mar. Dominant Species were Achnanthes minutissima, Aulacoseira granulata var. angustissima, Cyclotella stelligera, Cymbella minuta var. silesiaca, Coelastrum cambricum, Phacotus lenticularis and Synedra delicatissima var. angustissima. The dominance index ranged from 0.15 to 0.76, and the diversity was very wide as the range from 0.23 to 1.05, also evenness diversed such as two index. The cluster analysis by similarity appeared the high relations with Aug. and Sept. at station 2 and the index was 0.88. Of the relationships between standing crops and physico-chemical factors, it was not the regularity between standing crop and chlorophyll concentration, and ranged from 0.89 at station 2 to negative index at other stations. Of dominant species, the relationships between Cyclotella stelligera and chlorophyll concentrations were the positive relations as up to 0.65 at all stations except to station 4.