Two high-manganese steel compositions were prepared (low-carbon steel (LC): 0.05C-26.3Mn-4.88Si-4.13Al-Fe, and high-carbon steel (HC): 0.52C-29.8Mn-2.96Si-2.73Al-Fe). These were homogenized, hot rolled, heat treated, and finally subjected to tensile testing and other characterizations. The LC steel, with a duplex microstructure containing body-centered-cubic (bcc) and face-centered-cubic (fcc) phases, exhibited extremely poor room-temperature ductility. On the other hand, the HC steel, with a single-phase austenitic structure, showed a good tensile property due to twin-induced plasticity (TWIP) effect, with total tensile elongation as high as 56 pct. Structural analysis of steel LC was carried out and it was found that the bcc phase of steel LC was actually an ordered DO3 phase, which was identified as the principal reason of poor ductility of steel LC.