In greenhouse trial all tested cucumber cultivars differed in their response to downy mildew infection.Bingo, EL-Nimir and EL-Afdal cvs. were highly susceptible by 65.1, 60.2 and 53.3% respectively, while Magdi cv. was the least susceptible one (10.4%). In addition, the least susceptible (Magdi cv.) was found superior to all other cvs. with respect to greater plant height, fresh and dry weight, the percentage of flowering (87.4%), while fruit yield average was (42.7Kg) and gaving the minimum percentage of leaf infection  (9.1%). Total sugars were higher in the healthy plants of the highly susceptible cultivar than that in the least susceptible the P. cubensis infection decreased the total sugars in both cultivars. Due to P. cubensis infection the total phenols were increased in both cultivars tested. Total phenols were higher in leaves of the least susceptible cv. than that in the highly susceptible one. Total free amino acids content was higher in the healthy plant of the highly susceptible cv. than that of the least susceptible one P. cubensis infection decreased the total free amino acids in both cvs. Both chlorophyll and carotene contents in healthy leaves of the least susceptible cv. were lower than that of the highly susceptible one. P. cubensis infection decreased the content of chlorophyll and carotene in both cvs.