A novel quartz crystaE microbalance (QCM) biosensor for the determination of organophosphorus pesticide, such as dichlorvos (DDVP), has been fabricated with poly (3, 4-ethylenedioxythiophene)/Poly (4-styrenesulfonate) (PEDOT/PSS) and cholinesterase butyl (BuChE). It is found that PEDOT/PSS coated sensor exhibits the highest sensitivity for DDVP among PEDOT, polyvinyl pyrrolidone, polyvinyl alcohol and polyacrylic acid sensor series. The conditions to prepare PEDOT/PSS coated sensor are optimized. The sensitivity of the PEDOT/PSS biosensor with BuChE is higher than the others without BuChE The determitration of dichlorvos in the range from 400yg/g t0 2400 yg/g used PEDOT/PSS with BuChE coated biosensor was obtained.