This study was carried out to investigate phytoplankton community structure in the Naktong river from January to December in 1999. In water quality, average value of pH were 8.1, BOD 2.5mg/$ell$, COD 5.0mg/$ell$, Chl-a 41$mu extrm{g}$/$ell$, and $NH_4^+-N$ 0.08mg/$ell$, respectively. Phytoplankton were identified 42 genera 76 species. Among these, diatoms were 39 species(51.3%), green algae 25 species(32.9%), cyanobacteria 4 species(5.3%), dinoflagellates 4 repectively. The highest standing crops were 33,023 cells/$mell$ in February at the Mulgum and the lowest 79 cells/$mell$ in March at the Goryung. Also, Standing crops were increased with proceeding from middle part to lower part. Seasonal succession of phytoplankton represented that Stephanodiscus hantzschii was dominant species in winter, Cyclotella menaghiniana and Synedra acus in spring, C. meneghiniana, S. acus, Aulacoseira granulata var. angustissima in summer, and A. granulata var. angustissima and C. meneghiniana in autumn. Ecological important species were 4 species, that are Stephanodiscus hantzschii, Cyclotella meneghiniana, Synedra acus, and Aulacoseira granulata var. angustissima. In the community analysis, dominance indices ranged from 0.434(August, Namji) to 0.999(January, February, Mulgum) and diversity indices from 0.026(February, Mulgum) to 3.073(September, Namji). According to the similarity index among the stations, it was generally defind as two areas such as middle(Goryung, Namji and Samryangjin) and lower part(Mulgum).