A two-year field study was conducted at El-Giza Agricultural Research Station to (1) estimate heritability of powdery mildew (PM) resistance when disease incidence (DI) or disease severity (DS) was used as criterion for evaluating resistance, (2) assess PM resistance of ten flax genotypes (lines), (3) determine relationship between each of DI and DS ratings and agronomic or technological traits. Genotypes component of variance of DI and DS were highly significant each year indicating that extensive genetic variation for DI and DS were present within the tested genotypes. Heritability and genetic advance of DS were greater in both years than those of DI demonstrating  that considerable progress in breeding for PM resistance could be expected in current breeding programs when DS is used as criterion for evaluating resistance. DI ratings ranged from 72 to 100 % in 2019 and from 51 to 93 % in 2020. DS ratings were also high and ranged from 65 to 98 % in 2019 and from 44 to 97 % in 2020. None of the tested genotypes were significantly less susceptible than genotypes no. 3, but the genotypes were either as susceptible as genotypes 3 or significantly more susceptible. Genotype 3 was a promising genotype for commercialization for the following reasons: firstly, it was the least susceptible genotype. Secondly, it showed environmently stable performance. Thirdly, it showed superiority in some traits compared to some of the tested genotypes. No significant correlations were observed between disease intensity variables ( DI and DS) and agronomic or technological traits. Correlations among agronomic or technological traits were sensitive to changes in environmental conditions. Possible mechanisms of resistance to PM in flax were also discussed.