A mathematicaml odel of NO^-₃N runoff process from a forested mountain basin is developed, based on a tank model comprising a surface tank with soil water storage systems, and a ground water tank. The model takes account of three elementary processes of NO^-₃-N in the forested mountain soil. They are infiltration and advection, solute exchange between mobile water and immobiler egions, and nitrification by microorganisms in the soil. The process of former two are modeled as a function of the rainwater movement and soil moisture saturation calculated by the tank model.The latter process is modeleda s a functiono f soil temperaturec alculatedb y a soil temperaturem odel. The model is applied to annual observations in 1992 obtained in Shirakawa-tani fo restedm ountainb asin (main species Japanese cedar) located on Shikoku island in Japan. The model describes the seasonal change of NO^-₃-N streamwater concentration well, through a year.