Contents of enzymatically hydrolysable phosphorus (EHP) in particle and colloidal phases in Lake Taihu were analyzed. Colloid in lake water was separated by tangential flow ultrafiltration, which included the component with diameters from 1nm to 1000nm. The EHPs from Wangyu River, a large water channel connected Lake Taihu with Yangtze River, and from Lake Qiandao and Fuchun River were also studied. Mean of particle EHP (particle size larger than 1000nm) accounted 58% of total EHP, which was the main contributor in the water. Mean of colloidal EHP accounted 35% of total, which also shows a high ratio compared to the true dissolved phosphorus. During the investigation period, total phosphorus and EHP in Wangyu River were 0.216mg/L and 0.174mg/L, respectively, suggesting that there is a high ecological risk to pump too much Yangtze River water to Lake Taihu. In Lake Taihu, percentage of micro-size colloid in marcophyte dominated zone (Xukou Bay) was much higher than that in cyanobacteria dominated zone (Meiliang Bay).