This paper is a study of the environmental base flow of Keelung River from Yuan-Shan-Tzyy to Pa-Duu. In the modeling of habitat method (Rhabsim), we apply the hydraulic suitable curve of three species of native fishes including Rhinogobius candidianus, Acrossocheilus paradoxus, and Hemimyzon formosenum. The ecological base flow of Rhinogobius candidianu is 1.0 cms at the site, Acrossocheilus paradoxus is 0.1 cms, and Hemimyzon formosanum is 3.0 cms. For the computation of hydrological method, the 30% of the average daily flow is 3.0 cms. For the water flow Q_(75) to maintain the water quality, it is 1.98 cms at the hydrological station of Pa-Duu. It should be able to satisfy the demand of ecological flow.