Effect of ball diameter has been studied on the limestone and clinker samples under batch grinding conditions based on a kinetic model. Firstly, eight different mono-size fractions between 1.7 mm and 0.106 mm formed by a root 2 sieve series have been prepared. Then, S-i, and B-ij, equations are determined from the size distributions at different grinding times, and the model parameters (S-i, a(T), alpha, gamma and phi(j)) are compared for four different ball diameters (41, 25.4, 20 and 9.5 mm). The results show that the study on the effect of ball diameter on the grinding gives more different results than other investigations. The variation in specific rate of breakage with feed size is independent of ball diameters and kind of materials.