Abstract: The deformation resistance of medium manganese steel at different temperatures was measured on a Gleeble-3500 thermo-simulator system. Reverted austenite transformation in medium manganese steel during warm rolling was investigated by means of tensile testing by stages, scanning electron microscopy (SEM), electron back scattered diffraction (EBSD), and X-ray diffraction (XRD). It is shown that wa:rm rolling at 600℃ and annealing after hot rolling result in more reverted austenite, and excellent mechanical proper;ies are gained with 859 MPa strength and 36% elongation. In the earlier stage of tensile deformation, serrate flow stress behavior is obviously observed because of sustained TRIP effect produced by reverted austenite. During the deformation, reverted austenite with a larger grain size is prior to transform because its stability is poor. In the later stage of tensile deformation, serrate flow stress behavior disappears. Ultrafine grain ferrite and martensite undergo plastic deformation, and martensite strengthening and dislocation strengthening of ferrite become the main strengthening mechanism.