This paper dealt with experimentally the effect of $N_2$ and $H_2$ gas mixtures ratio in the fatigue characteristics of SM45C on Ion-nitrided. The specimen were treated water cooling after Ion-nitriding at $500^{circ}C$ and 5 torr. in 80% $N_2$and 50% $N_2$gas mixtures ratio in the atmosphere for 3 hrs. The hardness distribution and the depth of nitriding layer shows more increase in 80% $N_2$gas mixture ratio than 50% $N_2$. Ion-nitrided specim- en for 80% $N_2$gas mixture ratio show more increase infatigue strenght in the $>1.5{ imes}10^5$ cycles region than 50% $N_2$. In the $<1.5{ imes}10^5$cycles region, fatigue failure is due to cracking of the brittle nitrided case, and the propergation of the surface cracks into the core. But in the $>1.5{ imes}10^5$cycles region, it is found that cracks propagate from the non-metallic inclusions in the subsurface.