The effect of spermidine on induced resistance to powdery mildew in Cucumis sativus L.cv.‘Changchun mici’ seedlings was conducted,and the activities of 4 defense enzymes and expression of 3 defense gene were tested in cucumber leaves after treated with spermidine and inoculated with powdery mildew.The results showed that:(1)0.2～1.0 mmol·L^-1 spermidine expressed different induced effects on cucumber seedlings powdery mildew resistance,and 0.8 mmol·L^-1 spermidine had the best effects on cucumber seedlings and the induced efficiency reached to 55.3%.(2)The defense enzymes of cucumber seedlings such as peroxidase (POD),phenylalanine amonnialyase (PAL),chitinase and β1,3glucanase activities increased after spermidine inducement or powdery mildew inoculation.The increase of these enzymes activity treated with both spermidine inducement and powdery mildew inoculation were faster than those only treated with inducement.The expression of POX,PAL,PR1a gene enhanced after spermidine inducement or powdery mildew inoculation.In summary,the results indicate that spermidine enhance the expression of defense genes,increase the activities of defense enzymes,decrease disease index significantly,then increase cucumber resistance to powdery mildew.