Since 2000, environmental policies and regulations in Korea are rapidly changing to TMDL(Total Maximum Daily Load) and nonpoint source control. This is due to bad water quality in drinking water sources. Although many environmental facilities having high removal efficiency are constructed and applied in nationwide for controling various pollutants from wastewaters, the water quality in rivers is worse and worse because of nonpoint pollution. In fact, TMDL is a new environmental regulation controling total daily loadings from watershed areas. Actually, the nonpoint pollutant is originated from various landuses and its control is based on TMDL regulation. Therefore, this research is performed to determine the size of detention basin to control nonpoint pollutants from resort developing areas. The detention basin is one of best management practices, which is useful for controling pollutants and flooding from the developing areas. However, it should be designed and constructed with cost effective method. Recent 10 years rainfall data are used to determine the size of detention basin. The cost effective size is determined to 7.4mm accumulated rainfall.