The present investigation was carried out to examine virulence diversity among the 291isolates (seven populations) of Rhizoctonia solani AG-1 IA causing sheath blight of rice by sclerotia production and pathogencity studies on four rice cultivars representing different observed disease reactions, using detached leaf inoculation method in conjunction with mycelia growth measuring. Based on pathogencity test four tested cultivars were significantly differed from each other in response to each population of pathogen. In each population, all the test isolates exhibited varying degree of virulence on each four cultivars and were variable both in mycelial and sclerotial (number and weight) parameters. There was found one group in each population having more stable performance in virulence in a range of genetically different rice cultivars based on their virulence on each four cultivars and along with their coefficient of variation by cluster analysis method. According to these finding, six, two, five and one high-virulence isolates respectively from populations of Rasht, Tonekaboon, Amol and Golestan can be used to future greenhouse or field tests, selecting most suitable isolates for identification of accurate reaction of rice cultivars / lines for rice breeding programs in north of Iran.