A series of wet grinding experiments using calcite, pyrophyllite, and talc powder by a vertical type planetary ball mill, in which the size and distribution of grinding balls were varied with the Gaudin-Schuhmann equation, were carried out and the grinding rate constant K ' in equation of grinding kinetics was examined based on the grinding kinetics analysis of experimental particle size distribution of ground products obtained in various grinding conditions. The regression equation for the grinding rate constant K ' was expressed by an empirical equation involving the mean diameter of composite grinding balls (d) over bar (B) [mm], the median diameter of feed x(mo) [mum], and Bond's work index W-i [kW h/t] as follows: K ' = c(1)/(w(i)x(mo))(2)(x(mo)/(d) over bar (B))exp[-c(2)(W(i)x(mo)/(d) over bar (B))] where the values of the empirical constants c(1) and c(2) were 592, 0.0438, respectively, within the experimental ranges.