Results of experiments whose aim was to determine the rate of grinding of mineral materials with different susceptibility to comminution are discussed in the paper. Experimental materials were gabbro and sandstone. The experiments were carried out in a laboratory ball mill. During grinding the number (mass) of balls of constant diameter at constant feed mass was changed. The main objective of the experiments was to give a specific rate of grinding of particular size fractions of the mineral materials used in road making. An initial equation was the Gardner and Austin formula in a differential form for discrete values of the fractions, assuming an ideal mixing of the ground material. The grinding rate for selected size fractions of the ground material was described by a correlation equation. Parameters of the correlation equation and the effect of ball number (mass) on their values were specified. Additionally, a mathematical relation describing the change of a mean particle diameter in time, was given.