In recent years there has been an increasing trend within Australia to instal large diameter (> 9.0 m) AG/SAG mills. A number of these mills have been extensively surveyed by the JKMRC under a wide range of operating conditions. Using the JKMRC＇s AG/SAG mill model, the results from these surveys together with those from a large data base of pilot scale mills were analysed in terms of their breakage rate distributions. The variations in these distributions were found to be similar at both the pilot scale and in mills up to 9.6 m in diameter and were related to operating conditions such as mill speed, ball charge volume, ball size and ore feed size. The data indicated that the effect of increasing speed, ball charge or ball size in SAG mills was to increase the degree of coarse particle breakage, often with an associated reduction in the degree of fine particle breakage. The effect of increasing the feed size was found to be dependent upon the mode of mill operation ie AG or SAG, with AG mill throughput increasing as the feed F-80 was increased. The opposite was true for SAG mills providing the ball charge was in excess of about 5% by volume.