Concentration of colloidal phosphorus in Lake Taihu, China, was determined in this paper. Lake water was sampled in April, 2005. Cross-flow filtration technique was applied to extract colloid from the water. The result shows that colloidal phosphorus concentration in Lake Taihu varied from 0.017 to 0.029 mg/L. In Meiliang Bay, an algae type zone, it is 0.023-0.029 mg/L; while in Gonghu Bay, a grass type zone, it is 0.017-0.022 mg/L. The colloidal phosphorus account for 28.6% of total phosphorus (TP) in Meiliang Bay, and account for 39.3% of total phosphorus (TP) in Gonghu Bay, respectively. The true-dissolved phosphorus in Meiliang Bay is four times of that in Gonghu Bay.