The mechanical and deformation microstructure properties of the Fe–Mn–C TWIP steel was investigated by means of tensile experiment, in situ scanning electron microscope (SEM) and transmission electron microscope (TEM).The results showed that the sample has excellent mechanical with tensile strength of the steel is about 1140 MPa and the yield strength is higher than 480 MPa, while the elongation is above 57%, the true stress–strain curve from tension tests exhibited repeated serrations and its strain-hardening rate is constantly changing. It is found that there were different deformation mechanisms at different deformation stages result in the unique true stress–strain curve. Dislocation slip dominated the initial deformation and with the accumulation of deformation stress concentration reached the twin shear stress resulting in twin shear, which lead to TWIP effect. As the strain capacity increased continually, the parallel twins can no longer rotate and shear deformation occurred, which lead to the forming of shear bands. The intercoordination of slip deformation, twin deformation, and shear deformation mechanism make the TWIP steel show high strength and high plasticity.