In order to investigate the effects of ball diameters and contact loading rates on ring crack occurrence, static and dynamic contact tests were carried out using ceramic bearing ball. As the results, the scatter of ring crack initiation strength in contact test of same ball diameters is smaller than that of same size. Ring crack occurs at a distance from contact boundary with decreasing ratio of ball diameters. Initiation strength of ring crack is not evaluated by maximum tensile stress on the ball surface, because micro-defects of ball surface doesn't consider. Therefore, the evaluation should apply the stress intensity factor. Since the ring crack occurrences are not dependent on size effect, the scatters of strength are constant even if ball diameters change. In the case of dynamic contact tests, the strengths are increased with decreasing loading time. Then, the scatter is also small. Ring crack strength in dependent loading rates can be evaluated using stress intensity factor and cumulative effective hold time.
