On the basis of the analysis of time-varying particle size distribution of material, it was found that it can be described by a one-parameter Rayleigh function. The reduction ratio was defined as a ratio of the parameter of this function for a material being fed to the parameter of the function for a material being ground. The relationship describing the change of reduction ratio in time constitutes a mathematical model of the process kinetics. The model contains two constants: one determined the rate of grinding, while the other the boundary reduction ratio which is possible to attain in a given device.