Ardabil province is one of the most important areas in potato production in country and Rhizoctonia is the important diseases on potato in this area that cause canker on the roots, stolons and underground stems of the growing plants, cutting off growing buds and and black scurf on potato tubers .One of the most important aspects of management of this disease is determining the relationship between the severity of the disease and its economic loss. For determine the amount of damage caused by Rhizoctonia chanker during the growing season based on the crop area is cultivated, sampling of at least 5% of potato fields in two areas Ardabil and Namin with the same four conditions including cultivating the dominant variety, the same method of planting and irrigation and the area of more than one Hectare were taken from emergence of seedlings until harvest. So that during three stages including seedling, flowering and harvest, by random sampling from the plants, was determined the percentage of damping off at seedling stage, the chanker severity on the plant at flowering stage according to the Clive patternby scoring 1-9 and the severity of black scarf on tubers according to the Clive pattern by scoring 0-5(Clive james, 1971) and weighting the yield of ten random plants at the harvest stage. The relationship between the severity of the disease and the damage was determined by using the data obtained from the sampling by calculating the correlation coefficient and regression(using software pachage Eviews). Also, determined existance a significant difference between yields in the groupings based on the different degrees of contamination that was a direct scoring by using the analysis of variance and the method of estimating the regression model with a small dependent variable (damages) . According to the statistical results, the seedling damping off percentage was 2.91% in Ardabil potato fields, therefore direct damage caused by the disease due to reduced green area (with crop density of 40,000 plants per hectare) was 1164 plants per hectare. The disease was present on most farms with low to moderate intensity and the average severity of canker disease on the stem in the flowering stage was 3.65 in the Clive scale and the severity of black scarf on potato tubers was 2.12 in the Clive scale. The amount of damage caused by this disease was obtained by multiplying the number of non-green plants per in the yield average weight of a plant per farm. Regarding the yield average of each plant at harvest stage(1.26 kg), the yield reduce of Rhizoctonia canker disease due to seedling damping off was estimated 1.65 ton/hectare in Ardabi .