The areas for agricultural purposes in Korea are decreasing every year because of urbanization. However, it is still 17.6% of the total national boundary on 2008. Most of the rice paddy fields are located near the waterbodies which require lots of water during rainy season from May to September. Also lots of nitrate and phosphate chemical fertilizers are spread on the fields every year in order to supply the nutrients for vegetation. The excess nutrients is impairs the water quality of rivers and lakes when it is washed out from the fields. The Korean Ministry of Environment (MOE) adapted the new water quality improvement program, which is the Total Daily Maximum Load, to improve the water quality and to protect the aquaecosystems. The constructed wetland is one of the possible ways to treat the agricultural runoff. The constructed wetland on this study area was constructed by MOE in 2007 to evaluate the application of the constructed wetlands. Plant growth continues to increase during the summer until it reaches its highest biomass of 6,032 g/$m^2$ in August and September. More researches about sedimentation, vegetation, water balance, etc. were performed to evaluate the removal efficiency, to find the removal mechanisms and to make the guidelines for design and maintenance.