Abstract: The optimum condition for testing polyphenol oxidase （PPO） in poplars was established as follows： pH 4.0, reaction time 1 minute, reaction temperature 5-30 ℃. The effect of tebuconazole， carbendazim， emamectin benzoate and abamectin on the activity of PPO from poplars was examined in the optimum conditions. The results showed that PPO activity was enhanced in poplars treated by 200 and 400 mg／L tebuconazole,and PPO activity increased in poplars treated by 100, 200 and 400 mg／L carbendazim. Furthermore, PPO activity was induced by 400 mg／L emamectin benzoate， and PPO activity was induced in poplars treated by 400 and 800 mg／L abamectin. Therefore, the effect of different pesticides stress on PPO was different in poplars.