Abstract: The effects of annealing time on the microstructure and mechanical properties of a cold rolled medium-manganese TRIP steel 0.1C-7Mn were analyzed on a CCT-AY-Ⅱ heat treatment system for thin steel sheet. The microstructure and composition of the steel obtained through different annealing processes were investigated by means of scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, and energy dispersive X-ray spectroscopy. The amount of retained austenite in the steel was determined by X-ray diffraction analysis and the mechanical properties were measured by tensile testing. The steel annealed at 650℃ for 3 min has the best comprehensive mechanical properties, with the tensile strength of 1329 MPa, the total elongation of 21.3%, and the product of tensile strength and elongation of 28 GPa.%. It is believed that the high plasticity is provided by the TRIP effect of metastable austenite together with ultra-fine grained ferrite. The high strength is caused by martensite which is directly transformed from austenite in the cooling process during annealing and obtained during the tensile deformation attributed to the TRIP effect.