In eutrophic lakes, the sediment usually plays as a large sink for phosphorous, but its capacity to take up more phosphate is quite limited. In this investigation, phosphorous saturation of the top sediment in East Taihu Lake was studied and used as an index of lake eutrophication. Eight sample points were sited along a line across the lake, and the top 10cm sediment was collected. The top sediment samples was also taken from am old fish farming pond and a pen as contracts. One series of row samples was cut into peacs of lcm and analyzed for total phosphorous (TP) and reactive phosphorous (RAP) content. It was found that the content of TP in the top sediment varies from 0. 0465% to 0. 0993%.and RAP from 3. 48 mg/kg to 19. 84 mg/kg. The fish-pond sediment had the highest TP and RAP content among all the the samples,but there was no remarkable difference between pen sediment and lake sediment.Ten experimental systems were prepared by mixing 2-kg sediment with the lake water in plastic tanks of 90L volume. The tanks were covered, aired with a pump and kept in dark. Phosphate (53mg P) was added to each system in four times to detect phosphorous saturation of the sediment. The sediment samples from fish-pond and pen were already phosphorous-satura-tred, and the lake sediment samples were nearly phosphorous-saturated. After adding 53 mg phosphate-phosphorous in each system. phosphate level of the water phase became as hign as 200-300/xg/L (25-48 times as high as in the lake water),but TP content of the sediment only increased 0. 037-0. 077 mg/g in the sediment (0. 06%-0. 14% of content). This indicates that the lake is now very sensitive to phosphorous pollution. Good care has to be taken for keeping a phosphorous balance in the lake ecosystem.