Evaluation of water quality of Yeong San river and Seomjin river by using of newly simplified MBOD method was performed. Of course, thought that there is some differences between nutrient demands of heterotrophic bacteria and those of Algae which obtain it by photosynthesis, but it has little influence on evaluation of Algae Growth potential. The result of this study were as follows: 1) In both river, the value of Chemical analysis and MBOD method of inorganic salts reveals as nearly same result. 2) Though organic pollution of Seomjin river is less than that of Yeongsan but inorganic contamination is somewhat advanced BOD 2.8 ppm. and MBOD 340 ppm. in Seomjin river but BOD 22 ppm. and MBOD 480 ppm. in Yeongsan river. 3) Both river have tendency to reveal higher Nitrogen value is MBOD=MBOD -P<MBOD-N So phosphorus valus is relatively low, it acts as limiting factor. 4) The sampling site Yt, downstream of Kwang Ju river, which is heavily polluted, reveals such tendency as MBOD < MBOD-P $ll$ MBOD-N So we can guess that there $Y_1$ any other substance which controls growth of organisms. Consequently Nitrogen and phosphorus are prevented to flow-in so as to limit the development of red tide of Seomjin river and eutrophication of Yeong San river.