The newly discovered large magnetostriction in antiferromagnetic γ-Mn–Fe alloy is of interest to develop a new kind of ductile and low-cost magnetostrictive materials. This work presents a detailed study on the structures, magnetostrictive, and magnetic properties of a heat-treated Mn 42 Fe 58 alloy by X-ray diffraction (XRD), transmission electronic microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometer. The single γ-phase separates into a mixture containing a face-centered-cubic (fcc) γ-phase, a hexagonal close-packed (hcp) ɛ-phase, and a body-centered-cubic (bcc) α-phase after isothermal heat treatment for 24h at 1100°C. Such phase separation in Mn 42 Fe 58 alloy not only leads to a slight increase in lattice parameter a of the γ-phase and Néel temperature T N , but also results in an obvious enhancement in magnetization due to the presence of ferromagnetic bcc phase. The magnetostrictive performance, however, deteriorates accompanying with the phase separation.