The sensitivity of Mancozeb, Dimethomorph and their different formulations to Pseudoperonospora cubensis was determined and effect of the optimal formulation tested in this study. The results showed that each of the various chemical treatments has a certain inhibition to germination of sporangia, and the inhibiting effect enhanced as the concentration increases. The co-toxicity coefficients of each formulation were all above 100, and the formulations of these two drugs had synergistic effect. The best formulation among them was the mixture of 60% Mancozeb with 9% Dimethomorph, which EC₅₀ reached 42.57 ug/ml. Moreover, the best control effect to Cucumber Downy Mildew was 69% Mancozeb·Dimethomorph WP 133g/667m², and the effective dosage that we recommend is 100~133g/667m².