A duplex ultrafine microstructure in a medium manganese steel(0.2C-5Mn)was produced by austenite reverted transformation annealing(ART-annealing).The microstructural evolution during annealing was examined by scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray diffraction(XRD).Based on the microstructure examination,it was found that some M3 C type carbides appeared in the martensitic matrix at the beginning of the ART-annealing.But with further increasing annealing time,these carbides would be dissolved and finally disappeared.Meanwhile,the austenite lath was developed in the ART-annealing process and the volume fraction of austenite increased with the increase of the annealing time,which resulted in a duplex microstructure consisting of ultrafine-grained ferrite and large fraction of reverted austenite after long time annealing.The mechanical property examinations by uniaxial tensile tests showed that ART-annealing(6h,650 ℃)resulted in a superhigh product of strength to elongation up to 42GPa·%. A microstructural evolution during annealing was annealed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Based on the microstructure examination, it was found that some M3 C type carbides were found in the martensitic matrix at the beginning of the ART-annealing. With further increasing annealing time, these carbides would be dissolved and finally disappeared .Meanwhile, the austenite lath was developed in the ART-annealing process and the volume fraction of austenite increased with the increase of the annealing time, which resulted in a duplex microstructure consisting of ultrafine-grained ferrite and large fraction of reverted austenite after long time annealing. the mechanical property examinations by uniaxial tensile tests showed that ART-annealing (6h, 650 ℃) resulted in a superhigh produ ct of strength to elongation up to 42GPa ·%.