Stream water delivers nutrients from terrestrial ecosystem to coastal ecosystem. Land use and land cover in watershed influence nutrient concentrations and fluxes of stream water. In addition, climate change will affect nutrient leaching to stream water through changes of nutrient cycles in terrestrial ecosystem. However, combined effects of land use / land cover and climate changes on nutrient leaching has not been fully understood. We aimed to understand the relationship between land use / land cover distribution and nitrogen concentrations in stream water in a forest, farmland and wetland dominated landscape where productivity of marine resources in the downstream lake play a key role in the local economy. We also aimed to understand the possible consequences of the future land use / land cover changes and climate change in the watershed in terms of nitrogen supply from terrestrial to aquatic ecosystem. In Bekanbeushi watershed located in eastern Hokkaido, Japan, we conducted a field survey of stream water quality and GIS analysis to understand the relationship between land use and nitrogen concentrations in stream water. The combined effects of land use land cover change and climate change on nitrogen leaching in the watershed was simulated using forest landscape model, LANDIS-II with NECN extension (Scheller et al. 2011). In the model simulation for 2015-2050, we utilized several scenarios that describe the different combinations of the following changes: abandonment of pasture management, changes in tree species composition in forest plantation, and changes in climate factors (i.e., air temperature and precipitation). The nitrate concentrations in stream water were higher in the watershed where farmland occupies large proportion compared to the watershed where forest and wetland are the main land cover. The pasture abandonment strongly impact on nitrate leaching compared with the changes in species composition in plantation forest. The amount of nitrate leaching in pasture land was affected by land use change than that by climate change, while opposite was the case in forest. These results suggest that effects of changes in land use and climate were different among land use types. We also found that precipitation change was more responsible for the temporal variation of nitrate leaching in future than temperature rise in all scenarios. Our findings would help the local community make a decision for more sustainable management and environment conservation in both forest and agricultural land of the landscape level.