Abstract: An amperometric biosensor for the detection of organophosphorus pesticides in vegetables was developed. This novel bienzyme electode biosensor was fabricated with acetylcholinesterase (ACHE) immobilized by chitosansilica on the surface of the polythionine modified glassy carbon electrode. Under optimal experimental conditions. The inhibition rate versus the logarithm of dimethoate concentration was linear over the range of0.1μg/mL to 60μg/mL with the linear correlation coefficient of 0.9961. The detection limitation for dimethoate, calculated by inhibition rate of 10%, was 0.05μg/mL.