In the designing of bearing to use in internal combustion engines, the author has established an assessment method for needle roller bearing considering the slip ratio of needle rollers when operating. Damages sometimes occur in a bearing when continuously operating the engine although the bearing has a basic dynamic load rating. For an example, flaking occurred in the needle rollers during the continuous operation of the engine when a roller bearing was applied to the bearing that supported the gear which transmitted the driving force from the starter motor to the crankshaft when starting the engine. While the bearing is located on the crankshaft, the bearing is not directly subjected to the loads from engine combustion pressures. We therefore focused our attention on the bearing slip ratio as a factor that affects the bearing rather than the loads.After researching the factors that affect bearing slip ratio of the concerned part, it has been clarified through the studies reported in this paper that the slip ratio can be reduced by applying the following three measures. 1Control of crankshaft whirling2Reduction of radial clearance3Application of crowning on needle rollersThe actual reduction of slip ratio enabled prevention of flaking when continuously operating the engine. As the result, an optimum designing of bearing is possible by the bearing life assessment considering the slip ratio of needle rollers during engine operations.