A two-year field study was conducted in EL-Gemmeiza Agricultural Research station to : (1) estimate heritability to powdery mildew disease when disease incidence and disease severity were used as criteria to evaluate resistance. (2) assess the susceptibility of 12 flax genotypes to powdery mildew; and (3) determine the effect of the disease on yield and yield components. Disease incidence (DI) and disease severity (DS) were used to evaluate susceptibility of the genotypes to the disease. DI was associated with high heritability and with high genetic advance expected from selection. DS was environmentally unstable. Most of the tested genotypes did not show satisfactory levels of resistance to the disease. However, line 42 / 140 / 5 / 11, line 3 and cultivar Sakha3 were the least susceptible genotypes. These genotypes also showed superiority in some agronomic traits compared with the other genotypes. Many significant correlations were observed among agronomic traits each year. On the other hand, no significant correlations were observed between disease intensity variables (DI and DS) and agronomic traits.