The objective of this study is to analyze the relationship between stream water quality of TN (total nitrogen), TP (total phosphorus), and BOD (Biochemical Oxygen Demand) and TDI (Trophic Diatom Index) score determined by physico-chemical factors, biomass, and standing crops of epilithic diatoms, and to estimate the required amount of ecological streamflow for good water environment of Trophic Diatom. For the main stream of Chungju dam watershed of South Korea, total 100 field data of 3 years (2008 ∼ 2010) measured in May and September were used to derive the relationship between water quality and TDI. Trophic Diatom had high correlation (0.55 determination coefficient) with TN. Using the relationship, the required streamflow was evaluated by using the Soil Water Assessment Tool (SWAT) for good Trophic Diatom water environment through T-N water quality maintenance. The SWAT simulated 8 years (2003 ∼ 2010) stream discharges and T-N water quality along the main stream. From present garde C (score range: 30.0 ∼ 45.0) to grade A (score above 60.0) of TDI, the May needs additional streamflow of 63.1 ㎥/sec (+36.7 % comparing with the present streamflow of 172.0 ㎥/sec) at the watershed outlet.