The objectives of the present study are to clarify breakage properties of coals in ring-roller mills and to develop a mathematical mill model capable of predicting steady-state mill performance in full scale mills. It was found that ring-roller mills have a higher coal grinding capability than ring-ball mills with an equal grinding efficiency. It was also demonstrated that breakage characteristics of coals in the mill could not be predicted by the conventional Hardgrove grindability index, HGI, under low filling conditions. On the basis of basic experimental results, a mathematical model was constructed to describe the behaviour in the mill. The model proposed here is based on breakage parameters determined form a batch ring-roller mill and utilizing a new scale-up rule employing apparent grinding energy rates in the batch ring-roller mills as well as large scale ring-roller mills. Furthermore, empirical rule of classification were also added to the mill model. The validity of the model was verified thereafter in large scale mills.