High manganese steels show significant potential for industrial application due to their remarkable TRIP/TWIP effects at room temperature. The study on the TRIP behavior during warm deformation is important in controlling microstructures and properties of high manganese steels. In this paper, the microstructures, phase structures and reverse transformation of martensites to austenite in a high manganese steel which is composed of two types of martensites and austenite were investigated under warm deformation (100—500℃) by means of the determination of transformation temperature, calculation of phase diagram, microstructure observation, XRD analysis and EBSD orientation imaging technique. Results show that during compression above 300 ℃, TRIP effect disappeared and reverse transformation from martensite to austenite was enhanced. The transformation from bcc martensite to austenite was determined to be diffusive and no nucleation of austenite was needed. The warm deformation of austenite leads to the formation of coarse deormation twins and the mechanical stabilization of austenite, which suppressed the subsequent martensitic transformation during quenching. The austeniic grains in which reverse martensitic transformation completed at the latest, show mainly {110} and {100} orientations. In addition, hcp martensite could hardly edetected around bcc martensite, and the transformation of hcp martensite into austenite is regarded to be reversible and diffusionless.