Microstructures and mechanical properties of Ti-V micro-alloyed TRIP (transformation-induced plasticity) steel with different compositions were investigated by tensile test,scanning electron microscopy (SEM),transmission electron microscopy (TEM),X-ray diffraction (XRD) and thermodynamic calculation (TC).The results indicated that the steel exhibited high ultimate tensile strength (1079 MPa),sufficient ductility (28％) and the highest product of strength and ductility (30212 MPa · ％) heat treated after intercritical annealing at 800 ℃ for 3 min and bainitic annealing at 430 ℃ for 5 min.In addition,the change of volume fraction of retained austenite (VF-RA) versus tensile strain was measured using in-situ analysis by X-ray stress apparatus and micro-electronic universal testing machine.It was concluded that a-value could be used to evaluate the stability of retained austenite (S-RA) in the investigated Ti-V micro-alloyed TRIP steel.The smaller a-value indicated the higher stability of retained austenite (S-RA) and the higher mechanical properties of Ti-V micro-alloyed TRIP steel.