The bending fatigue strength (10⁵-10⁷ cycle) of gas nitrided or gas nitrocarburized SCM435 steels was evaluated. The steel formed compound layer on the surface exhibited higher fatigue strength than that with less compound layer. When the compound layer was mainly with γ′-Fe₄N phase, the strength was remarkably high. The yield stress and residual compressive stress of the compounds were higher than those of the diffusion layer, respectively. The compound layer may prevent initial crack generation. In both of ε-Fe₃N and γ′ compounds, multiple microcrackings on the specimen surface may reduce the stress concentration at the main crack tip and make the steel toughed. The γ′ compound showed the higher residual compressive near the surface stress than ε one, was plastically deformed, and was of fine grains so that the microcrack growth in the γ′ compound may be prevented. Therefore, γ′ compound showed the higher fatigue crack initiation resistance than that of the ε compound showing brittle crack growth, and then the nitrided steel with γ′ compound layer exhibited higher fatigue strength.