The objectives of the present study were to evaluate the resistance to powdery mildew of three flax collections introduced from Russia, Belgium, and Holland compared with a collection of Egyptian cultivars. Broad sense heritability (h²) and genetic advance expressed as a percentage of mean were also evaluated for powdery mildew severity, seed weight, and straw weight within each collection. Powdery mildew severity of Russian and Egyptian collections, seed weight of Russian and Dutch collections, straw weight of all collections except the Russian one exhibited high h² coupled with high genetic advance expressed as percentage of mean indicating that each of these characters is controlled by additive gene action and simple selection could be effective in improving these characters. Powdery mildew severity of each of the Belgian and Dutch collections showed high h² with moderate genetic advance indicating that the powdery mildew severity of each collection is controlled by non-additive gene action. Therefore, this character could be improved by intermating superior genotypes of segregating populations developed from recombination breeding. On the other hand, seed yield exhibited medium and low h² in the Belgian and Egyptian collections, respectively, and straw weight of the Russian collection also showed low h². These results indicated that each of these characters was strongly influenced by environmental changes, thus, it cannot be utilized as a selection criterion in breeding programs.