Abstract: The toxicity of bifenthrin to Aphis sp. was determined by the method of the residual film in glass tube. Carboxylesterase activity of Aphis sp. was measured. The Km and Vmax of carboxylesterase in populationⅠ and populationⅤ was studied. And the inhibition effects of 4 insecticides (chlorpyrifos, triazophos, methomyl and omethoate) to CarE in different populations were determined. The results showed that Aphis sp. had resistance to bifenthrin in different levels compared with the sensitive populationⅠ of Aphis sp.. LC50 of the sensitive population Ⅰ was 0.057 mg/L, and it was 4.344 mg/L in the case of population Ⅴ. The resistance of populationⅤ to bifenthrin was 76.2-fold to that of the sensitive population,which could be classified as high level resistance. The activity of CarE increased with the increasing of resistance levels. There was no significant different in Km between sensitive population Ⅰ and high resistant populationⅤ. However, the Vmax of resistant populationⅤ was 2.08-fold compared with the sensitive population Ⅰ. Inhibition effects of 4 insecticides to CarE of different populations were different, and I50 increased with the increasing of resistance level to bifenthrin. Carboxylesterase plays an important role in the resistance of Aphis sp. to bifenthrin.