Abstarct: The most prevalent way to eliminate Sulfur and to decrease the ash of coal with average size in coal washing plants is grinding to smaller sizes for further washing. In Tabas Coal Washing Plant, particles of 0.5-5 size, and 0-0.5 mm size, are processed through heavy media separation and flotation, respectively. In this thesis, the performance of industrial cage mill was evaluated in terms of circulation speed change, capacity, and water adding.Tri-flo heavy media separator which washes the 6-50 mm particles of raw coal, produces a middle product with 30-40 percent ash which is 1/10 of input feed capacity of the plant. In the present study, first the washability of the provided representative sample was recognized through middle circuit of Tri-flo. Then, the sample of the study was ground to -3 mm through a jaw crusher in the form of open circuit, and the amount of released coal by grinding was evaluated through sinking and floating method (for +0.5 mm particles) and ash testing (for -0.5 mm particles). The results of the study indicated that the grinding of middle product resulted in releasing 23 percent of coal with 10 percent ash, hence, in 2.3 percent increase in the total yield of the plant so the grinding of middle product of Ma’danju Mine with the 60 percent of critical speed of the cage mill results in the best yield by 57 percent coal release with 11.5 percent ash. Central Mine and Mine appeared to need more speed due to the interference of gangue and coal. However, due to the less fines production, 60 percent of speed was considered as resulting in the best yield, and the ratio of grinding at lower speeds in the presence of water equals with the grinding at more speeds in the absence of water.