Abstract: The microstructure of twinning induced plasticity (TWIP) steel after tensile deformation was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) analysis. The results show that there are 60% annealing twins in TWIP steel before deformation and 32% deformed twins after deformation. The grains with annealing twins deform firstly during tensile deformation. This deformation mechanism of twinning induced plasticity provides specimens' excellent elongation with no-necking phenomenon. The orientation of the first deformed twin originates from the annealing twin's. The interactions between twin and twin, twin and dislocation, and twin and sliding play an important role during deformation. Therefore the effect of twinning induced plasticity is the main deformation mechanism.