The photocatlytic degradation of chlorpyrifos using TiO2 through high-pressure mercury lamp has been investigated. The results showed that the optimum concentration range for TiO2 was 50100 mg/L. When initial concentration of chlorpyrifos was at the range of 580 mg/L, the kinetics for degradation followed the first order reaction in which the bigger the initial concentration, the smaller the degradation constant and the longer the half-life. The half-life of 80 mg/L chlorpyrifos was 27.18 min, 1.5 times longer than that of the 5 mg/L chlorpyrifos, chlorpyrifos and alkaline condition was more favorable for degradation of chlorpyrifos.