Experimental and theoretical studies have been conducted on the control of product particle size distribution by use of a closed-circuit pulverization system. The performance of a pin mill type pulverizer and a rotational blade type classifier were modeled by use of experimental data. The returning powder flow rate was taken into consideration and material balance equations of each size range were numerically solved by an iterative method. When the median particle diameter of the product is constant, it is necessary to set a low rotational speed of classifier and a high rotational speed of pulverizer for low return ratio region. The opposite condition is applied to the high return ratio region. Under the condition of constant median diameter of product, the standard deviation shows the minimum value when the return ratio equals one. For the return ratio less than one, increase of cut size of the classifier produces a large standard deviation. For a return ratio greater than one, the decrease of rotational speed of pulverizer is the main reason to the increased standard deviation of the product particle.