Abstract: The reasons for resistance of cucumber to P. cubensis are studied with the methods of microscope, transmission and scanning electron microscope, results are as follows:1. There is no distinct difference in stomata density and size on leaf surface between resistant and susceptible cultivars, but the fungous spots of P. cubensis are difference of germination on leaf surface. 2. The cytological changes in infected leaves:When resistant cultivars are infected by P. cubensis, the infected cells break down into particles rapidly, both cells and fungous die. The mesophyll cells between two infected spots largely multiply. In these cells, the number of chloroplast decreases and membrane system are destroied, cell walls thickened.There are a few very small necroses on leaf surface.When middle resistant cultivar is infected by P. cubensis, a few fungi trail among mesophyll cells and produce some sporangiophores and sporangia. The membranes system of mitochondria and chloroplast in the infected cells are destroied, then infected cells and all fungi die. Dying is slower than that of resistant cultivars.There are a lot of large necroses on leaf surface.When susceptible cultivar is infected by P. cubensis, large numbers of fungi trail among mesophyll cells,and mature fungi can produce many conidiophores and conidiosporangia. After infected cells break down into fragments gradually and die at last due to fungous parasitism, fungi infect adjacent healthy cells.There are a great number of large necroses on leaf surface, and necroses often join into larger ones.