Isotopic compositions were analyzed to grasp the altitude effect observed in and the discharge characteristics of river water in Yakushima Island, southwestern Japan. The isotopic characteristics of the river water are similar to those of precipitation in summer; a regression line for the river water is δ D = 7.68 δ18O + 10.06 and d values are in a range of 6.5 ∼ 16.1‰. Moreover, the altitude effects are -0.17‰/100 m for δ18O and-1.3‰/100 m for δ D irrespective of river drainage size. These altitude effects are well concordant with the values in the previous report in Japan, strongly indicating that the river water in Yakushima Island is scarcely affected by the processes of evaporation and mixing with groundwater having long residence time. This idea is not contradict with verifications using mass ratio of potential evapotranspiration in precipitation as an index. Such discharge characteristics of the river might be attributed to hydrological setting of Yakushima Island, i.e., lower permeability of granite and high amounts of precipitation up to 10,000 mm/yr in the mountainous area. Some river water samples in central-northern and western parts of the Island have apparently light isotopic compositions against the average altitudes of drainage basin. Considering landforms of Yakushima Island and meteoric conditions, such isotopic data would be caused by rain shadow effect in which vapor mass as a source of these river water samples has been brought across a ridge by seasonal southern wind.