Abstract: Wujiangdu reservoir is located at the mainstream of Wujiang river,the biggest tributary in the upstream of Yangtse River.The surface water was collected monthly near the dam of Wujiangdu reservoir from Oct 2003 to Sept.2004.The water sample and sediment cores were at the same station in (December) 2003,and April and July 2004 respectively.The concentrations of the dissolved silicon (DSi) in the water column and the porewater were determined,and the (Chl-a) concentrations of chlorophyll-a in the water column were analyzed.Other parameters such as temperature and dissolved oxygen were measured in situ.The result shows that the DSi concentration in the surface water ranged from 0.53 to 3.96 mg/L,with average of 1.74 mg/L; and the DSi concentration in the porewater is about 7 times higher than the overlying water.The DSi concentration in the stratified water bodies increases with their depths,compared with the increase of porewater,firstly,and then decrease with its depth.The good inverse relationship between DSi and Chlorophyll a suggests that the variation of DSi concentration in Wujiangdu reservoir is determined by the biological activities.