<p>In this paper, the effects of surface strengthening on high cycle and very high cycle fatigue behaviors of a CSS-42L gear steel are investigated by four-point bending and rotating bending fatigue tests. The results show that the S-N curve of the base material shows a plateau zone, while the S-N curves of surface carburizing and surface carbonitriding materials exhibit a continuous downward trend. The surface strengthening significantly increases the fatigue performance of a CSS-42L gear steel, and the high temperature reduces the fatigue performance for both the base and surface-strengthened materials. Scanning electron microscope observation indicates that, unlike the base material, there is a circular optical dark area around the crack origin for surface strengthening material. The carbide is found in the fatigue fracture surface of the surface-carburizing material, which is beneficial to suppress the crack extension. Finally, a fatigue life model is proposed based on the correlation between hardness and fatigue strength, which takes into account the influence of the surface carburization layer. The predicted fatigue life is in good agreement with experimental data.</p>