This paper presents the results of an optimization study to determine the optimal replaced wheel diameter when internal grinding tool steel. In this study, the minimum grinding time was chosen as the objective function. In addition, six main input parameters of the grinding process were selected for the investigation. The influence of these parameters on the optimal replaced diameter was analyzed. Moreover, a regression formula to calculate the optimal replaced wheel diameter has been proposed. It is convenient to use this formula because the formula is given as an explicit function.