Abstract: To clarify the toxicity of emamectin benzoate to 3 kinds of pests (Lepidoptera) and Trichogramma dendrolimi in Hexi region of Gansu Province, and coordinate the contradiction between pesticide control and biological control, the toxicity of emamectin benzoate to the third instar nymphs of Lepidoptera, including cotton worm, Asia corn borer, army worm and adult Trichogramma, was measured by using diet feeding method and residual film method. The results showed that the corrected mortalities of those four insects were raised with the concentration of emamectin benzoate increasing, the mortalities were more than 95% when the concentration went up to 0.500 mg/L. The median lethal concentration indicated that the toxicity of emamectin benzoate to T. dendrolimi was the highest among the four insects, the LC50 value was 0.078 mg/L, next in turn was Asia corn borer, cotton worm and army worm, and the LC50 value was 0.109, 0.120 and 0.127 mg/L, respectively. The toxicity of emamectin benzoate to T. dendrolimi was 1.40 to 1.63 times that of the other three. Thus, in order to reduce the harm of pesticide to natural enemy Trichogramma, the use of emamectin benzoate should avoid the emergence of Trichogramma in field, especially in IPM.