Abstract: In order to explain the effect of surfactant in microemulsification of pesticide and evaluate the bioactivity of pesticide microemulsion,the regularity of chlorpyrifos solubilized in the mixed system of surfactants were studied and the optimized formation of chlorpyrifos microemulsion was achieved by chlorpyrifos measurements of the maximum capacity solubilized in the solvent-surfactants system and the maximum concentration dispersed in different surfactants /water microemulsion systems, the bioactivity of chlorpyrifos 300 ME was also investigated on Helicoverpa armigera by laboratory assays and field tests. The results showed the maximum capacity of chlorpyrifos solubilized in the solvent-surfactants systems and the maximum concentration of chlorpyrifos dispersed in different surfactants/water microemulsion systems increased with the concentration of surfactants and the ratio of surfactants /water(m/m) increased respectively, but had different curves. The optimized microemulsion is composed of discrete spherical oil droplets which hydromechanical radius is about 25.9 nm. For laboratory assays, the bioactivity of chlorpyrifos 300 ME and 400 EC has no significant difference and with LC50 values of 500 mg/L and 561 mg/L respectively. For field tests of chlorpyrifos 300 ME, comparing with 400 EC at the same dosage, the control efficiency is not significant too. It indicated that it was the key to enough dosage of surfactant for the formation and stability of pesticide microemulsion and the bioactivity of pesticide was not enhanced significantly by the improvement of dispersion by microemulsification of pesticide.