Tensile deformation behaviors of Fe-0.10C-5Mn medium manganese steel were studied in a strain rate range of 10-3-500 s-1 using tensile testing machines. In-situ observation under scanning electron microscope and X-ray diffraction were adopted to investigate the steel. With strain rate increasing, yield strength and tensile strength of the steel of both high-speed and quasi-static tension increase, showing strain rate strengthening trends. Howerer, the strength at strain rate 0.1 s-1 drops to a valley. The elongation fluctuates greatly, increases first and decreases later. Strain rate affects the deformation mechanism of the steel significantly. On the one hand, the strain rate increasing aggravates the dislocation proliferation and entanglement, brings about a dislocation strengthening effect. On the other hand, it induces adiabatic temperature increment, leads to a "softening" effect. The higher the strain rate, the higher the stability of austenite, and the weaker the TRIP effect of the steel emerged.