In this study, the effects of nitriding methods and times on the fatigue strength of SACM 645 steel were investigated. The rotary bending tests were carried out to obtain and compare the fatigue strengths of plasma ion nitrided specimens and gas nitride specimen. The 70 hr. gas nitrided specimen had the highest fatigue strength of 1.05$ imes$$10^3$ MPa over the 40 hr., 70 hr. and 90 hr. plasma ion nitrided specimens, which had the fatigue strength of 3.48$ imes$$10^2$, 4.57$ imes$$10^2$ and 4.64$ imes$$10^2$ respectively. Also, the microhardness tests were conducted to measure the effective case depths. The plasma ion nitrided specimens showed much higher surface hardness values than the gas nitrided specimen overall.