Abstract: Norcantharidin showed obvious pesticide activity, researching the synergistic insecticidal activities was conducive to the development and utilization of the material. After mixing the chlopyrifos with norcantharidin and developing the 5% microemulsion of norcantharidin, the toxicological effect of two single pesticide and their mixtures and the 5% microemulsion of norcantharidin were tested by leaf dipping method. The results showed that the knockdown force of norcantharidin was higher than that of chlorpyrifos when tested individually: with LC50 values of norcantharidin and chlorpyrifos were 93.97 and 528.69 mg/L respectively after 48 h, the two pesticide showed an enhanced toxicity when the ratio of norcantharidin and chlopyrifos was 1:5, 1:2.5 and 1:1.25, the co-toxicity coefficient reached to 186.75, 181.72, 166.49(48 h). After adding 2%, 5% silicone to the microemulsion, the toxicity increased to 1.32 times and 1.68 times respectively. The mixture of norcantharidin and chlorpyrifos showed a certain degree synergistic interaction after 48 h. After processed the norcantharidin into microemulsion formulation, the toxicity increased obviously. After added certain quantity of silicone to the microemulsion, the toxicity increased also.