In order to clarify the natural purification potential of a natural wetland having free-flowing water, we performed a four-year study on such a wetland system which had been receiving for 12 years the domestic wastewater discharged from a residential area comprised of 45 households. The wetland's removal rate of organic matter throughout the four years ranged from 80% for COD to 95% for BOD, whereas the corresponding nitrogen removal rate was comparatively lower. Results indicate that NH4-N release from the bottom sediment and repression of nitrification are the main factors responsible for the wetland's low removal rate of nitrogen during winter. The wetland purification performance even in winter was determined as follows (g m−2 d−1): 2.2 BOD, 0.81 COD, 1.1 TOC, 0.10 T-N, and 0.023 T-P.