Two greenhouse experiments were conducted in the greenhouse of Agricultural Botany Department, Faculty of Agriculture, Kafrelsheikh University, Egypt, during two seasons (2012-2013 and 2013-2014) to study biochemical, molecular characterization and SEM investigation of susceptibility and resistance mechanisms of ten Egyptian wheat cultivars (Sakha 93, Sakha 94, Sids 12, Sids 13, Giza 160, Giza 168, Giza 171, Misr 1, Misr 2 and Gemmiza 11) inoculated with Blumeria graminis f. sp. tritici fungus the causal organism of wheat powdery mildew.This disease  one of most important diseases of wheat worldwide. Chemical and biological control methods are used routinely to control the disease. However, resistant cultivars are still the best control strategy. Consequently, characterization of susceptibility and resistance mechanisms is very important and required essentially. In wheat susceptible cultivars (Sakha 93, Sakha 94, Sids 12 and Sids 13), moderately susceptible (Giza 160), moderately resistant (Giza 168, Giza 171, Misr 1), resistant (Misr 2) and highly resistant cultivars Gemmiza 11) when inoculated with powdery mildew fungus, the percent of disease severity were significantly decreased in resistant cultivars compared with susceptible ones. Indeed the disease symptoms and electrolyte leakage were also significantly decreased in resistant cultivars. As well as levels of reactive oxygen species (ROS), such as superoxide (O₂^·-) and hydrogen peroxide (H₂O₂) significantly accumulated early 6 and 12 hours after inoculation (hai) in the resistant cultivars. Consequently, catalase (CAT), superoxide dismutase (SOD) and peroxidase (POX) enzyme activities were significantly stimulated at 24-72 hai. Significant increase of chlorophyll a and b concentrations was found. However, in the susceptible inoculated cultivars, the fungus was intensively colonized with huge quantity of conidia spores compared with the moderately resistant, resistant and highly resistant cultivars using scanning electron microscope (SEM). Molecular investigations using PCR (SSR) technique proved that the resistant gene Pm38 over expressed and accumulated in resistant cultivars not in susceptible ones. Expression of Pm38 gene was correlated with the resistance degrees. It can be recommended giving more attention to these new mechanisms of resistance to improve and find out new resistant cultivars which over expressed new resistant-mediated-ROS genes.