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The deformation mechanisms and the flow stress behavior of a medium-manganese high-carbon steel during cold deformation at a strain rate of 10?5 s?1 were explored using a universal testing machine, an X-ray diffractometer, a field emission scan-ning electron microscope and a high-resolution transmission electron microscope. The results show that continuous step-up serrated flow behavior appears after the yielding point, and the true stress–strain curve is roughly divided into five stages based on distinctive densities and amplitudes of serration. The strengthening mechanisms of the experimental steel involve Cottrell atmosphere, twinning-induced plasticity (TWIP) effect and transformation-induced plasticity (TRIP) effect. TWIP effect is the dominant deformation mechanism, and deformation twins formed by TWIP effect comprise primary, second-ary and nanotwins. Furthermore, TRIP effect arises in the local high-strain region. Carbon element plays a key role in the transformation of the deformation mechanism. A small amount of carbide precipitates around twin boundaries lead to the formation of local carbon-poor regions, andMd temperature and stacking fault energy of medium-manganese high-carbon steel are propitious to the occurrence of TRIP effect. In addition, the contributions of various deformation mechanisms to plasticity are calculated, and that of TWIP effect is the greatest.