Abstract: The effects of omethoate,imidacloprid and carbosulfan on the activities of polyphenol oxidase (PPO) and carboxylesterase (CarE) in cotton seedlings were investigated after treated for different period of time. The results showed that the effects on PPO and CarE activities in cotton seedlings related not only to the insecticides used, but also to the concentration of the insecticides and the treated time. The activities of PPO in cotton seedlings decreased by 21.6% after treated with 400 mg/L omethoate for 24 h. After treated with 50 mg/L imidacloprid for 120 h, the PPO activities increased by 47.3%. Using 200 mg/L carbosulfan treated the cotton seedlings for 72 h, the activities of PPO increased by 75.7%. The activities of CarE in cotton seedlings decreased by more than 70% after treated with 100, 200 and 400 mg/L omethoate during the period of 120 h . Using 50 mg/L imidacloprid treated cotton seedlings for 120 h, the CarE activities increased by 30.7%. But the CarE activities of control and the seedlings treated with 50,100 and 200 mg/L carbosulfan showed no significant difference at all the treated time.