Abstract: Room temperature low cycle fatigue, very high cycle fatigue, high temperature isothermal fatigue and thermo-mechanical fatigue properties of the as-rolled Mg-12Gd-3Y-0.5Zr magnesium alloy were investigated. The fatigue failure mechanism was analyzed. The results show that for room temperature low cycle fatigue and very high cycle fatigue, the failure was caused by the fatigue cracks resulted from the inclusions or large-sized secondary phases. The cracks initiated from the surface or sub-surface for low cycle fatigue, however, for very high cycle fatigue the internal cracking was the initiation. Both high temperaturei sothermal fatigue crack and thermo-mechanical fatigue crack originated from the surface. The interaction of cyclic slip and oxide inclusions was the main fatigue mechanism. The alloy had excellent tensile strength and fatigue strength from room temperature to 200 ℃. The opposed phase thermo-mechanical fatigue lifetime was longer than the in-phase one thermo-mechanical fatigue lifetime.