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Recent studies have provided a warning that fatigue failure can occur in thelife regime exceeding N=107 and at stresses lower than the conventional fatigue limit. Inthe present study, the mechanism of fatigue failure in extremely high cycle regime wasstudied on bearing steels, SAE 52100. Specimens having a longer fatigue life had aparticular morphology beside the inclusion at fracture origin. The particular morphologylooked optically dark when observed by an optical microscope and it was named theoptically dark area, ODA. The ODA looks a rough area when observed by SEM and AFM.In order to investigate the growth of ODA with cycles, the fatigue tests of multi-steploadings were carried out and the formation of ODA was compared with those of thespecimens tested under a constant amplitude loading. The condition for the critical size ofODA for the start of conventional fatigue crack growth mechanism was analysed byfracture mechanics. The critical stress intensity factor range ÄKODA can be correlated withthe threshold stress intensity factor range ÄKth for small cracks expressed by the areaparameter model.