Cucumber is one of the economically important vegetables in Iran which is extensively attacked by powdery mildew. Identification and application of disease resistant genotypes is an efficient method to escape the disease epidemic. A number of 23 greenhouse and field cucumber cultivars were evaluated and screened for powdery mildew resistance and putative resistant cultivars were identified based on percent Disease Index (PDI) measurement. In addition, leaf chlorophyll fluorescence was measured and analyzed in 5 cultivars selected from resistant, semi-resistant and sensitive groups. Based on the results of PDI measurements, a considerable variation between the cultivars was observed so that the cultivars classified to four groups including resistant, semi-resistant, semi susceptible and susceptible. Two greenhouse cultivars i.e. CLE and Green Magic with the PDIs of 7.78 and 8.33 were the most resistant and the two field cultivars Beit Alpha and SuperN3 with the PDIs of 31.11 and 31.67 were the most susceptible cultivars. Based on the ANOVA of chlorophyll florescence parameters, a significant infection effect on minimal florescence (Fo), variable florescence (Fv) and vitality index (Fv/Fm) and a significant genotype effect on Fo parameters was observed (P