Abstract: The effect of Mn content on microstructure and mechanical properties of medium manganese steel is investigated via metallographic observation, X-ray diffraction, transmission analysis and mechanical testing. The results show that, after intercritical annealing, medium manganese steels with different Mn contents(3.86%~5.40%) all contain the mixed microstructure of ferrite and austenite at room temperature. The content of austenite in the treated steel increases with the increase of Mn content, while the stability shows an opposite trend. With the increase of Mn content, The strength of the tested steel increases, while the percentage elongation after fracture and percentage reduction of area and the impact toughness of the tested steels decrease. The variation of mechanical properties of the tested steel with the Mn content is related to the existing form of Mn and the content and stability of reverse austenite.