Sclerotium rolfsii is a soil-borne fungal pathogen belonging to the Atheliales, causing root rot disease in a wide range of crops, ornamental plants, weeds and even forest trees. In order to investigate the population structure of this fungus in the north of the country, plants that showed symptoms of white rot in crown area were collected from Guilan, Mazandaran and Golestan provinces during the growing seasons of the years 2011-2016. A total of 90 isolates were isolated from 13 host plants, including crops and weeds. For Mycelial Compatibility Groups tests, discs of pure culture of each isolate were placed on a PDA medium at regular intervals and after 4 to 7 days the results were recorded. In compatibility mode, the mycelia of the isolates merged, but in the incompatibility reaction, the growth inhibition zone appeared and by creating a distinct boundary on the site of the collision of two isolates, the sclerotia was often formed around this area. The boundary width that formed between incompatible isolates was different and measured between 1-5 mm. In this study, nine MCG groups were identified. The MCG3 group (with 35 members) was the most frequent in terms of the number of isolates and the most diverse group in terms of host domain (due to infection of eight host species). Also this group (MCG3) was the only group that was isolated from specimens in all three provinces. Regarding to the fact that this fungus does not have any mitosporic stage, and basidium stage occurs very rarely in nature, as a result, the transfer of genetic material among populations is limited to the transfer of nuclei during the heterokaryosis process. Therefore, determination of mycelial compatibility groups is a suitable method for assessing genetic diversity and studying the status of fungal population in each region.