Dry grinding experiments were carried out with a planetary ball mill. Layer particles adhering to the mill wall and freely movable particles in the mill were separated from each other and changes in their weight fractions, size distributions and specific surface areas with grinding time were measured for various balls, feed sizes and powder fillings. Certain correlation between the decrease of grinding rate and the increase of coating particles was confirmed and a new empirical equation for expressing the change of specific surface area of product in batch grinding were derived.