Seasonal changes of the phytoplankton community was investigated from July 2000 to April 2001 at five stations in the Imjin River. A total of phytoplankton were composed of 193 taxa, belonging to 5 divisions, 145 species, 28 varieties, 1 form, 19 unidenfitied species. Of those the diatoms and green algae were present during the investigation periods more frequently than the other taxa. The phytoplankton standing crops varied from 89×10 exp (3) cells/ℓ at station 1 in October 2000 to 8,825×10 exp (3) cells/ℓ at station 3 in April 2001. Throughout the investigation periods, at station 1, there was low in the size of phytoplankton standing crops at large. Otherwise, at station 2, station 3 and station 4, we could see the maximum of phytoplankton standing crops in July 2000 and then phytoplankton standing crops decreased and it reached to the minimum in January 2001, and after then, it increased again. But exceptionally, we could see the maximum of phytoplankton standing crops in April at station 3. And also, at staion 5, we could see the similar variation pattern of phytoplankton standing crops at station 2, station 3 and station 4 with a little differences in seasonal variation pattern of phytoplankton standing crops. The major dominant species were Achnanthes minutissima, Anabaena sp., Aulacoseira ambigua, A. granulata, A. granulata v. angustissima, Cymbella affinis, Cylindrotheca closterium, Diatoma vulgare, Merismopedia glauca, Stephanodiscus hantzschii f. tenuis.