Abstract: The changes of related defense enzyme activity in the resistant reaction to melon powdery mildew were studied through determined the activity of POD, CAT, PPO after artificial inoculation. The results showed that activity of POD, CAT, PPO had no obvious difference between resitant and susceptible control.The three kind of enzyme activity all increased compared with their control after inoculation. CAT activity of resistant variety was higher than susceptible variety. POD activity is opposite. PPO activity of resistant variety was higher than susceptible variety until five days after inoculation.