Numerical simulation and experimental studies were conducted on the unsteady-state characteristics and stable operating conditions of a closed-circuit pulverization system. The performances of the Pulverizer and 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 solved numerically by an iterative method. The particles used were CaCO3 and fly ash with mass median diameters of 36 Pm and 16 pm, respectively. In case of fly ash particles, the performance of the pulverizer decreased with time. The corrected numerical simulation, which takes die drop of pulverizer performance into consideration, agreed with the experimental data. For the CaCO3 particles, the experimental data agreed with the numerical simulation. By defining the limiting diameters of the pulverizer and classifiers, stable operating conditions were proposed. The stable operating conditions were also analyzed in the figure for the two cases with and without the drop of pulverizer performance.