The promotion of heat island mitigation measures is an urgent need in Tokyo. This study simulated evapotranspiration, land surface temperature, and watershed averaged land surface temperature for each individual land-surface feature in the Upper Kanda River Watershed, implementing a water-permeable pavement for roads and green roofs as heat island mitigation measures. The simulation took an advantage of TET model that can express the amount of evapotranspiration, accounting for changes in soil moisture and permeable characteristics of individual land-surface features. As a result, it was found that the state of the soil moisture had a significant effect in the impact assessment of urban heat island mitigation measures.