Powdery mildew caused by Erysiphe necator, is one of the most economically damaging diseases of grapevine throughout the world. This is the first molecular biology study on this fungus in Iran. The aims of the present study were (a) to analyse genetic diversity between isolates of E. necator fungus in four main grape production regions in Iran, i. e., Qazvin, Alamoot, Takestan and Shahriar and (b) to obtain an overview on the probable mating types of E. necator isolates infecting grapevines in the mentioned regions. In total, 11 RAPD primers were used, amongst which 7 primers showed high level of polymorphism. Out of 20 isolates, only 4 isolates were of mating type + and the rest were of mating type –. For statistical analysis, bands with molecular size between 180-2900 bp and high degree of polymorphism were chosen for analysis. Cluster analysis was carried out using Jaccard’s similarity coefficient, based on UPGMA that classified isolates into 7 groups.