Abstract: This paper studies on insecticidal activity and Bioassays of Cantharidin(C10H12O4) which is insects’natural products and Demethylcantharidin which is cantharidin analogues. The bioassay result showed that Cantharidin(C10H12O4) and Demethylcantharidin have obviously insecticidal function, including stomach poisoning and antifeedant activity, to the 3th instar larvae of Plutella xylostella L. The main symptom are that activity of the tested insect decreases and the porepart of body turns black. The LD50 of Demethylcantharidin and Cantharidin is 819.5 μg/g and 8.6875 μg/g, repectively, the former is 94.4 times to the later. The AFC50 of Demethylcantharidin and Cantharidin in selective are 20.46 mg/L and 2.67 mg/L, repectively, the former is 7.66 times to the later. The AFC50 of Demethylcantharidin and Cantharidin in non-selective are 7.63 mg/L and 0.69 mg/L, repectively, the former is 11.06 times to the later. The synergists ZB-1 and ZB-6 have both positive synergism to the efficiencies of cantaridin and demethylcantharidin. By LC50 values the efficiencies of cantaridin and demethylcantaridin stomach poison to the.test insecticidal increased 3.47 and 3.28 times after it mixed with 8% of ZB-1 respectively; and increased 3.52, 1.97 times after it mixed with same density of ZB-6 respectively, Similarly.