The fatigue strength of materials subjected to dynamic loads is routinely determined by using an ASTM standard rotating beam test. The test specimen is prepared very carefully and tested under closely controlled laboratory conditions. The resulting endurance limit is multiplied by the modifying factors to approximate the endurance limit of mechanical or structural components. Among a variety of modifying factors, surface finish is one of the more important ones. The existing data for surface finish modifying factor for steels is limited to a maximum hardness of 33 Rc. For steel components with a hardness of approximately 50 Rc, the existing surface finish modifying factors cannot be applied. The objective of this research is to develop surface finish modifying factor curves for steels with hardness in the range of 50 to 55 Rc. The results of this investigation not only provide surface finish modifying factors for high hardness steel components, but also demonstrate that the existing factors, if extrapolated to 50 Rc and beyond, would lead to overly conservative (excess weight and cost) designs.