Abstract: Cold rolled sheets of 30Mn20Al3 high manganese non-magnetic TWIP steel were tensile deformed after solution treatment at 1000 and 800°C for 10minutes. Microstructure evolution was observed by OM and TEM. Results indicated that strain hardening rate followed different patterns along with changes in true strain under different deformation stage, and the strain hardening exponent increased with increasing true strain. Dislocation slip was the dominant deformation mechanism for small deformations. As deformation amount increased, deformation twins appeared and the main deformation mechanism was the interaction between dislocation and deformation twins or between deformation twins. Grain size increased with increasing solution treatment temperature. Microstructure observation demonstrated that the number of deformation twins increased with increasing grain size, resulting in a greater "TWIP" effect in coarse-grained specimens than in fine-grained specimen. This can be attributed to the dependence on grain size of the critical stress for twin formation.