In this project, the alternating torsion fatigue test was carried out to investigate the effect of the nitrided case produced by gas and plasma nitriding processes on the fatigue endurance of the JIS SACM 645 steel. The surface layers of the nitrided specimens exhibited hardness profiles in the range between 1000–1100HV0.1, and the white layer of the nitrided specimen was consisted of Fe3N and Fe4N. Wear test result indicated that the mass loss of JIS SACM 645 steel was greatly improved by nitriding processes. From the mass loss data, the wear resistance of the steel was significantly influenced by nitriding time. The fatigue strength of the 35C-GN48 nitrided specimen rose 51.9% to 632 MPa, which was the maximum fatigue strength in this study.