Species composition and dynamics of phytoplankton community were studied in the upper region of Sum River from June, 1993 to October, 1994. As a result, 222 taxa were observed, representing of 6 phyla, 7 classes, 15 orders, 34 families, 85 genera, 177 species, 26 varieties, 2 forma, and 17 unidentified species. The composition of occurrence species were as follows: diatoms are 46.8 %, green algae are 35.1%, blue greens are 11.3% and so on. Common species were Achnanthes sp., Cocconeis placentula var. klinorphis, Cymbella cistula, C. minuta, C. tumida, Fragilaria crotonensis, gomphoneis olivaceum, Gomphonema clevei, S. robusta var. splendida, Synedra ulna, Scenedesmus acutus var. acutus, s. quadricauda. The standing crops of phytoplankton varied from 169 cells/ml to 64,995 cells/ml. Maximum cell number was observed in June, 1993 and minimum in August, 1993. Species diversity, evenness and dominance index suggested that phytoplankton community structure of the upper region of Sum River were probably diverse and stable.