It is important to understand and evaluate the impacts of rice cultivation on water environment for sustainable agriculture because rice cultivation have both functions of water pollution and purification with environmental and cultivation conditions. This paper presents the impact of rice cultivation on water environment in view of nitrogen flows in water systems in Korea. A simple protocol was proposed to assess the potential amount of nitrogen outflow from paddy field. And potential amount of nitrogen input and output by water flow at paddy field were estimated with the protocol at the conditions of the nitrogen contents of irrigation water, amount of fertilizer application, and irrigation methods. Where irrigation water was clean, below 1.0 mg L-1 of nitrogen concentration, rice cultivation polluted nearby watershed. At the conditions of 2.0 mg L-1 of nitrogen concentration in irrigation water, 110 kgha-1 of nitrogen fertilizer application, and flooding irrigation, rice cultivation had water pollution function, but it had water purification function with intermittent irrigation. At the conditions of 3.0 mg L-1of nitrogen concentration and flooding irrigation, rice cultivation had water purification function with 110 kg ha-1 of nitrogen fertilizer application, but it had water pollution function with 120 kg ha-1 of nitrogen application. Where irrigation water was polluted over 4.5 mg L-1 of nitrogen, it was evaluated that rice cultivation had water-purifying effect, even though the amount of nitrogen application was 120 kg ha-1. Key words; Rice paddy, Nitrogen, Irrigation, Water pollution, Nutrients