Abstract: The ultrathin section was studied by the observation of electron microscope. The results showed that the peritrophic membrane(PM)presented a more obvious multilayer structure, and the surface was evenness and smooth in the 4^th instar larvae of Hyphantria cunea Drury. After treated 120h by using the mixture of Bt and Chlorbenzuron on larvae of H. cunea Drury, many white and black spots were formed on PM. Then the spots expanded to patchy pores. Finally thin and brittle, easily broken or fester, inelastic PM fragments were formed. It lead to destruction of the PM and death of larval. The test proved that the damage effect of the treatment of the mixture of Bt and Chlorbenzuron on the PM of the 4 instar larvae of H. cunea was the fastest, and which of the single agent of Bt and Chlorbenzuron on the PM was the second. At the same time, the tests of the value of LT₅₀, LT₁₀₀ of the 4^th instar larvae of H. cunea and the air-spraying control effect by using the mixture of Bt & Chlorbenzuron and its single dose proved that the mortality rate of the mixture was higher than that of the single dose, and the rate of death toxicity of the mixture was faster than single agent. The results showed that the mixture could quickly destroy the PM of the 4^th instar larvae of H. cunea, and the toxic effect of the treatment was significantly enhanced.