In order to evaluate the global warming effect on the water quality in a reservoir, we conducted sensitivity analysis as to the change of air temperature as well as its seasonal pattern at Urayama Reservoir in Arakawa River basin using a 3-D reservoir water quality simulation model of ELCOM-CAEDYM. The results of simulations showed that the increase of air temperature caused more water temperature increase in epilimnion than hypolimnion. Especially the increase of air temperature in summer caused more water temperature increase in epilimnion, which would easily lead to the water quality problems. The increase of air temperature also caused the decrease of total Chl.a in the reservoir. Especially the increase of air temperature in winter caused significant decrease of chlorophytes and cryptophytes. Cyanobacteria also decreased by the increase of the air temperature because the water temperature in summer became much higher than its optimum temperature.