Abstract: [Objective] To provide data for toxicity assessment of avermectin on diamond back moth, Plutella xylostella (Linnaeus), the leaf protection role of avermectin was studied. [Method] The toxicity was tested by leaf-dipping method and the corrected mortality and leaf protection rate at 24h、48h or 72h were calculated.[Results] For the 2nd instar larvae, the LC50s at 24h、48h and 72h were >10.00mg/L、2.87mg/L and 0.56mg/L, respectively. The PC50s at the respective duration were 0.25 mg/L、0.82mg/L and 1.43mg/L, respectively. Meanwhile, for the 3rd instar larvae, the LC50s at 24h、48h and 72h were >10.00mg/L、6.20mg/L and 3.00mg/L, respectively. The PC50s at the respective duration were 0.25 mg/L、0.82mg/L and1.43mg/L, respectively. [Conclusion] Based on the leaf protection rate, the effect of avermectin on the tested insects came out quickly after tratment. But the corrected mortality should reach a high level while the toxicity became stable, otherwise the leaf protection effect would not be well enough.