A selective withdrawal system (SWS) is essential for the conservation of water quality in a dam reservoir. To make clear the desirable operation of SWS, we applied numerical simulations (the one-dimensional and the three-dimensional water quality simulation models called DYRESM-CAEDYM and ELCOM-CAEYDM, developed by CWR) to the Tono dam reservoir which is now under construction. The results of 1-D simulations indicated that SWS operation using two intake gates simultaneously could be more desirable for the daily operation considering the water temperature, DO and Chl. a in the reservoir. But the results of 3-D simulations indicated that the surface intake operation could release more suspended solid from the reservoir during flooding period.