In this study, the effect of mill speed was investigated on different particulate pumice samples at batch grinding conditions based on a kinetic model. For this purpose, S i and B i,j equations were determined from the size distributions at different grinding times, and the model parameters (S i , a T , , , and phi j ) were compared for three different mill speed (fraction of mill critical speed; 65%, 75%, and 85%). The effect of fraction of mill critical speed (phi c) on the grinding for model parameter a T found to be different for four different pumices. Additionally, in this study it was found that optimum grinding occurs at phi c=85% for all pumice samples, in contrast to the 75% of critical speed of the ball mill.