Sheath blight disease, caused by Rhizoctonia solani AG-1 IA, is one of the most serious fungal diseases of rice, worldwide. Rice resistance is a relative phenomenon and disease response varied (due to complex quantitative inheritance and high environmental diversity), so an accurate disease assessment would be difficult. To detect a reliable reaction of rice in a high level of disease development, a new inoculation method was developed, using Mycelial Mass (MM) of R. solani, produced in PD-broth along with application of a water sprayer system. Seedlings from three local and from two improved cultivars (Anbarbo, Ghasroldashti, Ahlemi-Tarom, Khazar and Bahar, respectively) were transplanted into the field. After 45 days past, tillers from each plant were sealed using rubber bands, then inoculated by a virulent isolate of pathogen, in three methods of: mycelial mass, colonized Rice Grain/rice hull (RG) and Freshly Infected rice stem (FI). Inoculated plants were kept moist until the last scoring time by through of a water sprayer system, above the tested plants in the field. Disease scoring was recorded 15 and 25 days following inoculation and at the end of dough grain stage, using the Relative Lesion Height (%RLH) method. Detailed disease assessment introduced the FI method as appearing with a low level of disease development, and of highly varied results. But MM and RG methods were of less varied results within each cultivar and at different times of evaluation. The most conspicuous vertical disease development was observed (among all tested cultivars) at the second evaluation stage and at the end of dough stage (the most susceptible stage of rice growth) in the MM inoculation method. Therefore, the plants were divided into three and two groups, respectively, and were found different from grouping by RG and FI methods (DMRT 5%). Therefore under water sprayer system, MM inoculation method can be used for the measurement of rice reliable reaction to sheath blight disease in the field. Anbarbo, Ghasroldashti, Ahlemi-Tarom and Bahar were identified as moderately resistant with the same statistical group from first screening test, but in this study they fell in different groups and their reliable tolerance were appeared after optimization of the test condition by development of disease, so Ahlemi-Tarom showed very susceptible reaction.