In an effort to investigate water pollution characteristics of Juam lake, water samples were collected from three sites (Sites A, B, and C) of Oenam stream which is a typical tributary of rural watershed in the lake and analyzed for N concentration and the corresponding isotope ratio (δ^(15)N) of NO_3^-. Concentrations of NO_3^- were not dramatically different among the sites; 0.8±0.2 mg N L^(-1) (range: 0.0~4.3 mg N L^(-1)) for Site A, 1.1±0.2 mg N L^(-1) (0.0~4.3 mg N L^(-1)) for Site B, and 1.1±0.1 mg N L^(-1) (0.1~2.6 mg N L^(-1)) for Site C. Meanwhile, δ^(15)N tended to decrease with river flow; it was highest for Site A (45.5±5.3‰) followed by Site B (19.7±2.0‰) and Site C (8.7±1.5‰). Such high δ^(15)N values of NO_3^- in Site A suggested that NO_3^- derived from livestock feedlot (specifically livestock excrete of which δ^(15)N is higher than 10‰) is the predominant pollution sources despite mountainous area occupied the most of land-use in the watershed. Using the two-sources isotope mixing model, it was estimated that the contribution of cropping activities (i.e. fertilization) became greater in down-stream area (Sites B and C) due to the higher agricultural land-use than the up-stream area (Site A). Particularly, during the active cropping season, the low contribution of organic pollution sources indicated that domestic sewage was not the predominant pollution source. Therefore, it was suggested that agricultural sources such as livestock farming and cropping rather than mountainous and residential are the dominant sources of water pollution in the study area. These results could be effectively utilized in elucidating water pollution sources in rural areas and selecting water management practices.