Abstract: The resistance to downy mildew ［Pseudoperonospor cubensis(Berk.et Curt.)Rostov.］ of cucumber can be induced when the cucumber seedlings are treated by the copolymer of chlorine and potassium ions with 2g/kg,5g/kg,10g/kg,15g/kg and 20g/kg concentration at the stages of cotyledon and first true leaf.In the experiment of a natural disease infection,the disease in treated plants is observed 3 to 6 days later.The mean rates of disease plants which are induced by copolymer of chlorine and potassium ions with 2g/kg,5g/kg,10g/kg,15g/kg and 20g/kg concentration are 25.26%,26.63%,31.46%,39.12% and 33.38%lower than those of the control,respectively;the mean rates of disease leaf are 22.89%,28.08%,34.02%,38.36%,and 36.59%lower.The mean disease indexes are 18.32%,28.97%,34.80% 35.89% and 36.77% lower with significant differences between the treatments and the control.The mean relative immunization efficiencies are 31.99%,43.75%,51.23%,53.43%and 53.70%,respectively.In experiment of an artificially inoculated disease infection,the mean disease indexes are 38.51%,44.86%,40.81%,43.00%and 43.33%lower than those of the control,respectively;and the most significant differences between the treatments and the control are attained.The lag period of the induced resistance to downy mildew of cucumber is 2～4 days,the latent period is 7～9 days,the protection period of induced resistance is 27days.In addition,the study shows that the optimum concentration of the copolymer of chlorine and potassium ions are 10g/kg and 15g/kg.