TWIP (twinning induced plasticity) steels possess very high plasticity and high strength. It has been pointed out that deformation twinning plays an important role in controlling the deformation behavior, which divides grains into nano--scale layer--like structures to result in high strain-hardening rate or the so--called “TWIP” effect. The formation of deformation twins is affected by deformation temperature, strain rate, pr--deformation and grain size. The generation of deformation twins in austenitic steel with low stacking fault energy (SFE) is closely related to grain size. However, the relationship between strain hardening rate and grain size in TWIP steels has yet to be clarified, which is important for optimizing the parameters of solution treatments. In the present paper, the specimens of a typical TWIP steel with grain sizes of 7, 13, 30 and 63 μm were fabricated through solution treatments at different temperatures. Mechanical properties were measured by tensile tests, and microstructure evolution was observed by OM and TEM. The results show that the strain-hardening exponent rapidly increases with increasing true strain when it is less than 0.2, but levels off in the subsequent process of deformation. The relationship between strain hardening rate and true strain consists of two stages for the specimen with small grain size and three stages for the specimen with large grain size. Microstructure observation demonstrated that the number of deformation twins increases with the increase of grain size, induced to greater“TWIP”effect in the coarse-grained specimen than in the fine-grained specimen. This can be attributed to the dependence of the critical stress for formation of deformation twins on grain size of σ_T=σ_T0+K_T}d^-A.