In this study, we investigated the correlation between the number of cyanobacterial species and the water quality factors after installation of Nakdong river weirs. Principal component and Factor analyses of water quality were also performed. The results of correlations showed that there was no significant water quality with the cyanobacterial concentration for the nutritive salts but when the NO₃ /NH₃ ratio is around 7 ~ 11 and the N/P ratio is about 27, cyanobacterial populations were abundant. There was a significant correlation with pH as important water quality factors for the growth of cyanobacteria. It is known that cyanobacteria grow in pH 8 ~ 9 range because cyanobacteria fixes HCO₃ ? into the intracellular carbon dioxide through carbon concentrator process using HCO₃ ?. The results of this study showed that the cyanobacterial population proliferated in a similar pH range of 8.6 to 9.6. In the anaerobic state, NO₃-N is continuously decreased and NH₃-N is eluted within about 6 days, so that the nitrogen elution of the sediment is not active in the anaerobic state. However in the same anaerobic condition as phosphorus dissolution occurs actively, eutrophication of nutrients and nutrients in water can be maintained. In the downstream region of Nakdong river, algae were more frequently detected than in the upstream region and the blooms of algae species seem to be influenced by the higher pH and the influx of the phosphorus from the upstream tributaries. Nutrient concentration was higher in the downstream region than in the upstream region due to the effects of tributaries Geumho river and Nam river in Nakdong river. The downstream is rich in nutrients and seems to maintain favorable conditions for the occurrence of cyanobacteria.