Discharge grates are one of the most important parts of SAG mills. These grates prevent balls and unbroken ores from discharging the mill and they have a significant effect on the mill filling and size distribution of the discharge flow. To consider the effect of openings of the grates on the overall efficiency of the comminution operation, the variation of the grates shape and size were closely monitored for a period of 5600 hours at the Sarcheshmeh copper concentration plant. Using measured values the changes in the amount of open area, the thickness of grates plates, the width of grates openings with respect to their radial position and grate plugging were determined. The results indicated that 1% increase in open area causes 0.4% increase in the circulating load of the circuit. The useful operational time of the grates was found to be 5000 hours beyond which the efficiency decreases.