Abstract: A desinged medium-Mn transformation-induced plasticity (TRIP) steel was treated by a new heat treatment, the content and stability of retained austentite were studied, the microstructure and mechanical properties of the steel were analyzed. The results show that the volum fractions of retained austenite contents in the medium-Mn TRIP steel after annealing were all over 39%, and they almost transformed into martensite during deformation process and thus increased the plasticty and stregth of the steel. The tensile strength(σb), elongation δ and σb·δ respectively were over 100 MPa, 30%, 30 GPa·%, and volume fraction of retained austenite was 51.14% after annealing at 630 ℃.