The resistance of 10 varieties of cucumber (Cucumis sativus L.) to powdery mildew, caused by Sphaerotheca fusca, was evaluated by a leaf disk assay. Leaf disks (10 mm in diameter) were removed from fully expanded leaves and then placed in petri dishes containing 0.16% water agar amended with benzimidazole. Leaf disks were inoculated by dropping a 10 μl of conidia suspension. Conidiophore formation of powdery mildew was the greatest at 25℃. The response of the host to powdery mildew, based on the inoculation onto disks of the first leaf, highly correlated with results obtained from harvesting stage of cucumber plants in greenhouse test (r = 0.99**). It is indicating that a leaf disk assay may precisely predict the response of cucumber plant to S.fusca.