The grinding rate characteristics of hard fine materials was investigated with a laboratory scale jet mill. The variations of product size distributions of synthetic diamond, alumina and quartz sand with the number of passes were described by using a grinding kinetics approach for various values of the grinding air pressure and the feeding rate. As a result, the grinding rate was found to decrease with increase in the hardness of comminuted materials from quartz to alumina and then to increase slightly for synthetic diamond due to its low impact strength. This trend is more pronounced for all materials with higher gas pressure and lower material feed rate. Thus, it is proved experimentally that the grinding characteristics of very hard materials even around 1, μm with the jet mill still fits the rate equation of comminution but the effect of operational conditions becomes low.