In the present study, the effect of heat treatment temperature and time on the microstructural and mechanical properties of medium manganese advanced high strength steel with 5 wt% Mn and 0.1 wt% B were investigated. Three temperatures were selected as annealing temperatures based on the stability of the austenite phase (stability temperatures of 50, 70 and 90% of the austenite phase). Mechanical characterization was performed using a uniaxial tensile test and phase and microstructural studies were performed by XRD and SEM. Alloy ingots were hot rolled, followed by cold rolling. The cold rolled sample had a martensite structure before the annealing process and the annealing process at all three temperatures resulted in the annealing of the initial martensitic structure. With raising annealing temperature from 680 °C to 710 °C, and consequently increase in residual austenite, tensile strength and ultimate elongation increased. Due to the additional annealing of martensitic matrix, yield stress decreased. The superior properties were resulted from the sample annealed at 710 °C for 20 min which led to tensile strength and ultimate elongation of 960 MPa and 22%, respectively. Increasing the time from 10 min to 20 min at annealing temperature of 710 °C, led to the rising of ultimate elongation and residual austenite. However, further annealing time (30 min at 710 °C) brought about reduction in austenite stability due to the dissolution of Precipitate phases Accordingly, the austenite phase turned into martensite during cooling and the mechanical properties of the sample significantly worsened.