Abstract: The effects of 5 benzoic acid compounds on polyphenol oxidase(PPO) of Spodoptera exigua were studied, when they were partially purified by 40% saturated (NH4)2SO4 and Sephadex G-100 gel filtration. The PPO is a key enzyme which takes part in melanin formation, sclerotization, recognition system in the development of insect, The results indicated that p-hydrobenzoic acid was the strongest inhibitor with I50 0.23 mg/mL, and the second compound was 5-methoxysalicylic. In addition, the results also showed that the PPO activity was inhibited by p-hydrobenzoic acid through mixed-competition with Ki=0.46 mg/mL,Kis=0.02 mg/mL,and 5-methoxysalicylic was competitive inhibitor with Ki=0.87 mg/mL.