Rhizoctonia solani (Telemorph: Thanatephorus cucumeris ) is a soil born plant pathogenic fungus with a wide host range and worldwide distribution that causes damages to the field crops, fruit trees, pasture and ornamental plants. The most common symptoms of root canker disease which is caused by Rhizoctonia is the black sclerotia on the surface of potato tubers, necrotic lesions on the roots, stems, and under ground parts of potato plants. The disease is very important in certified potato seed production and in addition to quantitative damages, it is effected the quality of the product and decreases its value in market. One of the important ways to control this fungus, which is nowadays considered, is the use of hypovirulent isolates containing viruses. Mycoviruses or fungal viruses are the kind of viruses that can replicate in fungal cells and so far more than 100 species of fungi belonging to different taxonomic groups have been reported. Most of these viruses have dsRNA genome, usually hidden in the fungal host and causing changes in their morphological, biochemical and pathological characteristics. The main purpose of this study is the detection of dsRNA in isolates of R. solani as a potato pathogen and study of its effect on morphological and pathological function of this fungus. The sampling were done in two years, 2007 and 2008, several times from infected potato fields in Isfahan province samples were collected and finally 55 isolates were obtained, their anastomosis groups and number of nuclei were determined. The dsRNA extraction from these mentioned isolates was performed by using cellulose column and after treatment with DNasel, RNaseA and S1Nuclease, dsRNA pieces with about 0/8 and 3 kb size, identified in E1-10 and E1-12 isolates respectively, that were belonging to Azadegan plain and also four ieces with size of about 0.8, 1.2, 3 and 22 kb in E4-3 isolate and two pieces with size of about 0.7 and 1 kb in E3-4 isolate, both belonging to Sepid Dasht (plain) were identified. The fragment in E1-10 isolate, amplified after purification from gel, using random PCR and K1, K2 primers and the bands were cloned and sequenced. The sequence of this isolate compared with the sequences in GenBank by using the Blast search tool in NCBI database, no similarity was seen in the sequences. Results from pairwise alignment of this sequence with multiple dsRNA sequences which were from GenBank and comparing them with pairwise alignment together showed that the above fragments could have a viral origin. Finally some isolates without dsRNA were compared with isolates containing dsRNA to evaluate the effect of dsRNA on morphological traits of colonies, mycelial growth and pathogenicity in R. solani . These isolates did Key words : dsRNA, R. solani, virus, fungi