Abstract: The objective of this study is to survey the resistance level of peach aphid Myzus persicae(Sulzer) in Hohhot and Wuchuan area, and investigate the insect resistance mechanism. The resistance of M. persicae to seven pyrethroids was determined by bioassay, and the carboxylesterase activity of this pest was also studied. The results show that relative to the sensitive population, Hohhot and Wuchuan populations have developed the highest resistance levels to cypermethrin(74.11 and 549.4, respectively). Hohhot population developed moderate resistance to bifenthrin(16.03), and kept susceptible to fenvalerate, beta-cypermethrin and beta-cyhalothrin(from 1.00 to 4.00). Wuchuan population has developed moderate resistance to deltamethrin(10.14), fenpropathrin(26.46) and bifenthrin(33.74), and demonstrated susceptible to beta-cypermethrin, fenvalerate and beta-cyhalothrin(RRM. persicae.Wuchuan and Hohhot populations illustrated significantly higher activity than the sensitive population. Carboxylesterase activity of Wuchuan, Hohhot and sensitive populations were all strongly inhibited by triazophos, with IC50 value of 0.54, 1.07 and 0.93 nmol/L, respectively. Bioassays showed that triazophos increased the toxicity of both beta-cypermethrin and deltamethrin when they were applied as a mixture, which showed that the combination application of triazophos and those two pyrethroids have the potential in the control of M. persicae in Wuchuan and Huhhot areas.