The effect of grain size on the flow stress in TWinning Induced Plasticity (TWIP) steel was investigated via the X-ray diffraction (XRD) measurements of dislocation density. The results indicated that the hardening behavior of fine grained samples (mean grain sizes in the range of 2.1-3.8μm) can be described as typical dislocation interactions. However in coarse grained samples (mean grain sizes in the range of 4.7-38.5μm) where extensive mechanical twinning occurs, another strengthening mechanism is required. Consequently, the effect of grain size on the flow stress parameters of the proposed equation was considered and it was found that in the fine grained samples, the Holloman analysis can describe the hardening behavior. However, in coarse grained samples, a second hardening term due to the strengthening effect of mechanical twin boundaries needs to be added to the Holloman equation.