River water temperature has been studied from various view points such as biological or ecological. Even in hydrology, river water tern perature has been treated only as a factor of water quality, and there are few investigations about changes in river water temperature with precipitation-runoff process besides snow-melting. In this paper, changes in river water temperature during a rainfall-runoff event were studied in the small vegetated watershed which is located about 20 km northeast of Mt. Fuji and has a drainage area of 0.12km2. Under a heavy rainfall of typhoon on Aug. 19-22, 1981, river water temperature was measured together with air temperature, soil tempera-ture, groundwater temperature, groundwater level, discharge, rainfall and surface runoff. As a result, it was observed that river water temperature rose suddenly about 0.2_??_0.5°C when surface runoff occurred and hydrograph reached its peak. On the other hand, the peak of groundwater level lagged behind rises of river water temperature. Groundwater tempera-ture and soil temperature kept constant. There were no remarkable changes in air tempera-ture during that time. Therefore, the cause of this phenomenon was attributed to a direct runoff accompanying thermal transfer. Two simulation models of changes in river water temperature with rainfall were present-ed. One model takes heat balance and mixing of water body into consideration, and the other model is based on a tank model. The former model represented that if direct runoff component occupies about 10% of total runoff, river water temperature rises about 0.2_??_0.5°C. Hydrograph separation by using specific conductance showed that direct runoff was nearly equal to 10% of total runoff at the peak of hydrograph. The latter model also proved that rises of river water temperature occur due to direct runoff.