In internal peripheral grinding processes the wheel cutting surface diameter is determined from the known work surface diameter and grinding wheel and work piece ratio D-b/D-o. Generally in internal grinding process, literature recommends different values for choice of ratio D-b/D-o. Investigation in this article shows that the choice of ratio D-b/D-o is important because there is an optimal field of ratio D-b/D-o in which the roughness of grinding surface is minimal.