Abstract: The sublethal effect of abamectin (AVM) and beta-cypermethrin(CPM) on the carboxylesterase (CarE) in diamondback moth (DBM), Plutella xylostella (L.) was investigated. The results showed that the specific activity of CarE in DBM increased in abamectin susceptible strain(XH-S)and decreased in abamectin-resistant strain(AV-R) after the treatment with sublethal concentration of AVM and CPM. The effect of AVM and CPM sublethal concentration on the affinity of CarE to α- or β-naphthyl acetate is varied. After treated with AVM and CPM, the Km value of CarE in XH-S strain showed no distinct difference from that of the control, In the AV-R strain, the Km value of CarE after treated with AVM is significantly higher than that of the control but no distinct change after treatment with CPM.