Abstract: The pathogen causing powdery mildew on Brassica napus L. in Shaanxi province, was indentificated by morphological characterisation and phylogenetic analysis. Conidiophores of pathogen were unbranched. conidia were brone singly, hyphae, cylindrical, or ellipsoid-cylindrica. No chasmothecia were found. The ITS region of rDNA was amplified with universal primers and sequenced. The collected five strains and the two Erysiphe cruciferarum strains fell within a single clade with 96% bootstrap support by established the parsimony tree. Based on phylogenetic analysis and morphological characters of the anamorph, we identi?ed the collected five strains as Erysiphe cruciferarum in Shaanxi province.