The seasonality of nitrogen export is essentially influenced by seasonal changes of both hydrological conditions and biogeochemical or plant physiological situations. Majority of the studies on this topic were conducted in climatically limited regions under marine west coast climate receiving 700 to 1400 mm precipitation with small seasonal variation. Dry soil, low groundwater level and flow situation occur typically in summer growing season under seasonally constant precipitation condition. Nitrogen export in this region decreases remarkably in summer, and this has been explained by high plant uptake and low water discharge from slope (soil) system to the stream. Contrasting to these cases, in the monsoon Asia and Oceania having high precipitation in summer, high nitrogen export is often observed in summer high flow season. In order to evaluate the effects of climate condition on nitrogen export from the forested ecosystems, we examined the processes generating the seasonal patterns of stream nitrate concentration in several forested catchments in the Asian monsoon region, and found that hydrological control was more important than the biogeochemical processes in plant-soil systems. Through the comparisons among forested catchments in US and Japan, the importance of seasonality in hydrological processes as the transportation system was emphasized.