The paper describes a methodology to simulate product particle size distribution of an industrial scale VertiMill (R) engaged in regrinding duty. Using survey data, the time-based population balance model has been utilized to simulate mill product particle size distribution. A sub-model relating mill power to particle residence time is developed and applied to the population balance model to predict mill response in different operating conditions. The result showed that the new time-based population balance model applied to the VertiMill (R) is capable of predicting product particle size distribution with a change in mill power, feed size distribution, mill feed rate, and slurry solids concentration.