Abstract: The influences of microalloy element niobium (Nb) on the stacking fault energy and mechanical properties of Fe-22Mn-0.4C alloy were analyzed. The stacking fault probability of Fe-22Mn-0.4C alloys with different Nb contents was determined by X-ray diffraction (XRD) profile analysis. The result shows that, with the increase of the Nb content, stacking fault probability decreases and the stacking fault energy of the alloy increases. During tensile deformation, the dominated deformation mechanism turns from transformation induced plasticity (TRIP) gradually into twinning induced plasticity (TWIP). Different deformation mechanisms play roles in the alloys with different Nb contents, which lead to changes in mechanical properties and stacking fault probability.