Abstract: The effects of Mn on the microstructure morphology,phase transformation and mechanical properties of medium manganese wear resistant steel were investigated by DIL805 A thermal analyzer,scanning electron microscopy,electron back-scattered diffraction,transmission electron microscopy and mechanical testing. When the mass fraction of Mn increases from 3% to 9% by a 2%increment,the volume fraction of austenite at room temperature gradually increases. The tensile strength and hardness decrease especially at the mass fraction of Mn from 5% to 7%. The orientation relationship between martensite and austenite varies. The morphology and type of martensite change from lath α martensite whose substructure is mainly dislocations to bunchy laminar α martensite and laminar ε martensite whose substructure is mainly dislocations,twins through phase transition and stacking faults.