Twinning-induced plasticity (TWIP) steels which have a high strength and high elongation use a twinning phenomenon during deformation. TWIP steels with more than 25% Mn can easily reveal the good combination of high strength and ductility with the conventional manufacturing processes without severe plastic deformation and accelerated cooling. However, the steels could not have an appropriate stacking fault energy for twinning with decreasing Mn content. In the present study, it was tried to make steels with lean Mn, that is, Fe-18Mn-1.5Al-0.6C (18Mn TWIP) steel and Fe-12Mn-2Si-0.9C (12Mn TWIP) steel and tested at various temperatures (25℃~ 600℃) and strain rates (10-4/s ~ 102/s) to investigate the effect of temperature and strain rates on the mechanical properties of lean Mn steel. Strength and ductility of 18Mn TWIP steel were increased with decreasing the strain rate at the range of 10-4/s ~ 10-1/s, but those of 12Mn TWIP steel were decreased with decreasing the strain rate at the range of 10-4/s ~ 10-1/s. However, the effect of strain rates over 100/s on strength and ductility of those steels was not significant. Strength and ductility of steels increased with decreasing test temperature at the range of 25℃ ~ 600℃. The deformation mechanisms of lean Mn TWIP steels were studied with the samples deformed with different amounts of strain using the fixture capable of controlling the deformation during tensile test. The volume fraction of twinned grain of 18Mn and 12Mn TWIP steel was increased with increasing strain and the stress also increased with increasing twin formation. Therefore, UTS is directly related with the volume fraction of twinned grain.