Abstract: Bacillus amyloliquefaciens LJ1 that was isolated from soil,possesses strongly biocontrol capacity against cucumber powdery mildew.The result of field trials showed that LJ1 is powerful in controlling cucumber powdery mildew.The 14-day control efficacy of LJ1 against powdery mildew in cucumber is 83.45%.In order to explore the biocontrol mechanism of LJ1,cucumber seedlings were sprayed with 100-fold supernatant of LJ1 fermentation broth and then the activity trend of superoxide dismutase(SOD),polyphenol oxidase(PPO),phenylalanine ammonia lyase(PAL) and salicylic acid(SA) in cucumber leaves were determined.Meanwhile,the quantity of fungi in cucumber rhizosphere was also detected.The results showed that all resistance index have a jump process at certain time point and the activity increased significantly comparing with that of the control group after being treated with supernatant of LJ1;Moreover,the quantity of fungi that could be cultured from the soil were drastically reduced and after 7d treatment.The results of present study suggested that some components in the supernatant of LJ1 fermentation broth could induce systemic resistance in cucumber seedlings and the induction-secreted active substances in cucumber rhizosphere is broad spectrum against fungi.