This paper presents an optimization study to determine the optimum replaced grinding wheel diameter to achieve the minimum machining time when external grinding grey cast iron. In this study, several main input process parameters including the initial grinding wheel diameter, the grinding wheel width, radial grinding wheel wear per dress, the total depth of dressing cut, and the wheel life were investigated. The influence of these parameters on the optimum replaced grinding wheel diameter was analyzed by using Minitab software. In addition, a regression formula to determine the optimum replaced grinding wheel diameter when grinding grey cast iron has been introduced.