Through analysis of the data obtained from the plant test in the Kamioka-Shikama concentrator, which includes a 2,700mm dia. multiplied by 3,600mm length rod mill and a 3,900mm dia. multiplied by 5,700mm length ball mill in a closed circuit with 2 or 3 hydro-cyclones of 600mm in diameter, a comminution mathematical model has been deduced from making use of a selection function under the assumption of plug flow material transport. Making use of the optimum models, the comminution in the mills and classification in the cyclones have been simulated in a wide range of operating conditions to obtain information on the conditions of the plant. The results of the simulations indicate that it is possible to gain higher throughput in the mill-cyclone circuit without changing the characteristics of the cyclone overflow.