Abstract: An excess of soluble phosphate in a river is one of the significant pollutions ruining the river water quality. Two estuary rivers with natural mangrove wetlands developing on their banks, which are few in Guangzhou area, were selected in this study for determining the capability of nature mangroves to decrease the soluble phosphate in the river water. Analyses of the water samples taken from the Shiziyang river downstream of a firepower station and that of Caochuancun river, indicate that (1) mangrove wetlands have a great capability to decrease the amount of soluble phosphate in river water; (2) This capability is decided by the size of the wetlands occupying the areas of a riverbed. With the same riverbed section the more size of a riverbed a mangrove wetland occupies, the more ability it gets to reduce the soluble phosphate in the water.