Mankyeong River system is the main stream of Chollabuk-do province, flowing across Chonju and Iri-City in the western district of Korea. The authors were investigated the freshwater algal community and its several environmental factors on summer, 1991. The environmental factors were air temperature(23.0-34.0˚C ), water temperature(22. 6-28.5˚C), pH(6.72-8.07,), dissolved oxygen (0.67-7.9mg/l), biological oxygen demand (0.82-18.27mg/l), salinity (0-0.5%) and electric conductivity(59-950 umohs/cm), respectively. The freshwater algae 214 taxa were classified and composed of 5 classes, 11 orders, 5 suborders, 22 families, 7 subfamilies, 68 genera, 181 species, 27 varities and 8 forma, and totally, 2.938 ×10^7cells/l were counted. In classification and relative abundances, 97 taxa(45.33%) of Chlorophyceae and 1.918×10^7cells/l (65.2%) of Bacillariophyceae were maximal, respectively. Totally, Synedra ulna is the dominant species, other main species are Melosira varians, Nitzschia palea, Cymbella tumida during this study. Biotic indices were dominances(0.192-0.403), H`values(0.78-2.23), eveners(0.46-1.39), similarities (0.135-0.536), pollution indices (1.2-2.4), pollution tolerance scores (0-4 degree). From above results, it suggests that the saprobity of the mankyung river and its tributaries were ranged from β-mesosaprobic to polysaprobic. And also, Synedra ulna was common species and 6 taxa including Cymbella tumida, Nitzschia palea and Melosira varians were regarded as the pollution indicators.