1. Studied on the seasonal variation of the water quality and distribution of phytoplankton in both of the South-Han river from Yang-su-ri to Yeo ju and the North-Han river from Yang-su-ri to Chun cheon during annual in 1969. 2. Comparing with the water quality of the South-Han river and that of the North-Han river according to the seasonal variation, the seasonal mean value of the South-Han river water quality, as for the Alkalinity, NH₃-N, NO₂-N, NO(-3)-N, C1^-, COD, Total hardness, Cahardness, PO₄^(-3), I₂-Consumed, SiO₂, F, SO₄^(-2), Nor.Bac, in summer season, Alkalinity, NO₃-N, Cl^-, Total hardness, Ca-hardness, PO₄^(-3), F, SO₄^(-2) in autumn season and also Alkalinity, NH₃-N, NO₂-N, NO₃-N, Alb.-NH₃-N, Cl^-, Total hardness, Ca-hardness, PO₄^(-3), Fe, F, SO₄^(-2) in winter season are higher than that of the North-Han river. The each seasonal mean value of Mn and SiO₂ in both rivers didn`t find any significantly difference. 3. The annual mean value of NO₂-N, Cl^-, PO₄^(-3), NH₃-N, F, SO₄^(-2) and pH in the South-Han river water quality are higher than that of the North-Han river water quality. Especially in the South-Han river the annual mean value of Alkalinity, Total hardness and Ca-hardness are twice and more higher than that in the North-Han river. 4. As for the annual mean value of SiO₂, Fe, Mn, NO₃-N, COD, Alkalinity and BOD didn`t find any significantly difference in both of the North-Han river and the South-Han river water quality. 5. In the view point of distribution numbers of phytoplankton, Navicula sp. and Cymbella sp. were predominated in the North-Han river and Melosira sp. and Naicula sp. were predominated in the South-Han river.