Abstract: The toxicity of acetone extract of Cinnamomum zexlanicum Nees and its effects on several esterases in Heliothis armigera (Hübner) were studied by leaf immersion method and ultraviolet spectrophotometry. The results showed that the acetone extract of C.zexlanicum had a good activity to H. armigera. The LC50 values and the LC95 values were 5.16 mg/mL and 27.26 mg/mL after 72 h of treatment; the corrected mortality were 69.8% and 80.2% after 48 h and 72 h of treatment at 13.33 mg/mL concentration and the corrected mortality was 64.0% after 72 h of treatment at 6.67 mg/mL concentration. The results also indicated that acetone extract of C. zexlanicum significantly inhibited esterase, carboxylesterase, alkaline phosphate esterase and acid phosphate esterase activities of H. armigera under various acetone extract concentrations of 5.0 ,4.0 ,3.0 ,2.0 and 1.0 μg/mL;the higher the treatment concentration, the heavier the inhibition.