The grinding rate constant, in the widely accepted first-order expression of grinding rate, is one of the important factors required to evaluate a grinding process particularly for its initial grinding stage of various mill types. In this study, we conducted grinding tests on silica glass, limestone and gypsum using a laboratory ball mill, and measured the grinding rate constant of the feed size reduction. We investigated the effects of feed size and ball diameter on the grinding rate constant of the materials used when the mass of balls, mass of feed, and the mill's rotational speed were constant. The results indicate that the variation of the grinding rate constant with feed size of the materials can be expressed by revising the equation proposed by Snow. For the grinding rate constant of each material, empirical equations were developed to express it as a function of feed size and ball diameter.