Abstract: Resistance of six field populations of Mythimna separata (Walker) collected from Beijing to five different types of insecticides was tested by leaf dip method in laboratory. The results showed that compared with the relative susceptible strain of M.separata, the six field populations exhibited different level of resistance to different types of insecticides. As to chlorantraniliprole, emamectin benzoate and chlorpyrifos, the six field populations of M.separata showed susceptible or low level resistance, with resistance ratio 1.314-4.213 fold, 1.000-4.385 fold and 1.083-5.936 fold, respectively. While to chlorfenapyr and lambda-cyhalothrin the resistance ratios of the six field populations of M.separata were 1.355-20.80 fold and 1.748-13.98 fold, indicating susceptible to moderate level resistance, respectively. It was suggested that in order to delay the generation and development of insecticide resistance in M.separata in Beijing, alternative and ratational application of insecticides between chlorantraniliprole, emamectin benzoate, chlorpyrifos and chlorfenapyr or lambda-cyhalothrin is necessary.