Potato black dot disease, caused by Colletotrichum coccodes, is a common disease in many potato planting areas in Iran. This pathogen causes yield loss in field and tuber rot in storage conditions. Genetic diversity and pathogenicity of C. coccodes was unknown in Iran. In this study, C. coccodes was isolated from potato plants with black dot symptoms from Ardebil, Isfahan and Hamadan provinces. Amongst 108 isolates, 48 isolates were studied for vegetative compatibility groups and aggressiveness. Ten VCGs including four multi-member groups (IRN-VCG1, IRN-VCG2, IRN-VCG3 and IRN-VCG4) as well as six single-member groups were detected. Isolates of Ardebil were grouped in IRN-VCG1 and IRN-VCG2 with 50 and 12.5% frequency and other isolates were single-member. Three groups of IRN-VCG1 (55%), IRN-VCG2 (13%) and IRN-VCG3 (13%) were constituted for Hamadan province isolates. Isfahan isolates were grouped as IRN-VCG1, IRN-VCG2 and IRN-VCG4 at 25, 19 and 12 percentage respectively, and 44% of isolates were not assigned to any of the four VCGs. The aggressiveness of the isolates were investigated using potato tuber decay assessment and root inoculation method. Most of isolates were able to decay potato tubers, while all isolates colonized the roots and were considered as aggressive isolates. Comparison of root and tuber inoculation methods showed no relationship between tuber rot severity and microsclerotia spreading in roots and stems. There was no correlation between VCGs and virulence while some correlation was detected for VCGs and geographical origin of the isolates. This study demonstrated that there is a significant VCG diversity in C. coccodes.