In this research the effect of feed characteristics on AG mill performance of Gole-Gohar iron ore processing plant was studied. For this purpose, 11 surveys in industrial scale were conducted on the circuit. At each survey for 2 hours, samples from AG mill feed and product were collected and mill data such as feed rate and power draw were recorded. Also for the evaluation of the effect of feed size distribution, in 3 surveys, feeder conveyor was stopped and all material on 25 meters of the belt were collected. SAG mill power index (SPI), was determined as the index of feed materials hardness. The result showed with increasing SPI, the product particle size decreased. A harder feed material produces more critical particles in the mill which elevates the mill volume content. It is expected that with increasing ore hardness, abrasion and attrition breakage mechanisms dominate and the product will be finer. Also the results showed, the SPI indexes increased with increasing the ratio of coarse particles (+100mm) to medium particles (-100+40mm).