Nitriding treatment is the technology to create compound layer and diffusion layer on the surface of steel. However, compound layer has low fracture toughness, so micro cracks tend to occur on the surface of nitrided steel. Therefore, in this research, surface defects of various dimensions (20-100 μm) were artificially introduced into nitrided steel and their influence on fatigue limit was investigated. To evaluate the fatigue limit of nitrided steel with surface defect, a new evaluation equation focusing on the plastic region at the crack tip and parameter model was proposed. The relationship between defect size and fatigue limit can be predicted by this equation using the threshold stress intensity factor range of macro crack and the fatigue limit of non-defect specimen. The predicted fatigue limits were in good agreement with the experimental fatigue limit. Thus, the usefulness of this equation was demonstrated.