The effect of Al contents(0∼5wt%) on the mechanical properties such as tensile behaviors and highcycle fatigue properties in a austenitic Fe-25Mn-0.5C alloy was investigated at room temperature. The increase of Al content in this alloy system reduced the austenite grain size and the amount of twin. The phase was a single austenite, which was not affected by the addition of Al. The yield strength was increased according to the increase of Al content. It is believed that this increase of yield strength results from the grain refinement and solid solution hardening. The ultimate tensile strength depends on the quantity of fine mechanical twin and solid solution hardening of Al. The total elongation decreased with increasing the Al content. It is considered that this variation of elongation is related to the quantity of the fine mechanical twin. In the high-cycle fatigue test. The specimen without Al which had an excellent tensile properties showed high fatigue-resistance. This high fatigueresistance was due to the formation of the mechanical twin which decreased crack propagation rate.