Microstructure evolution during ART-annealing (austenite reverted transformation annealing) of 0.2C-5Mn steel processed by austenitation at different temperatures was examined by SEM, TEM and XRD. It was demonstrated that the initial microstructures resulted from austenization at different temperatures strongly affect the microstructure evolution during followed ART-annealing, even the ultrafine grained ferrite/austenite duplex structure with about 30% austenite could be obtained after long time ART-annealing in all cases. Austenization in the intercritical region (between A c1 and A c3 ) gave a duplex structure after quenching, which was nearly not affected by followed annealing process. However, high temperature austenization (above A c3 ) resulted in a full martensite structure after quenching, which gradually transformed into a ferrite/austenite duplex structure during the following annealing process. Based on the analysis of austenite fraction and carbon concentrate, it was found that not only carbon partitioning but also manganese partitioning in the austenite affected the stability of austenite and even dominated the development of lamellar ferrite and austenite duplex structure during intercritical annealing with different times. At last an austenite lath nucleation and thickening model was proposed to describe the microstructure evolution of medium mangenese steel during ART-annealing. Microstructure evolution during ART-annealing (austenite reverted transformation annealing) of 0.2C-5Mn steel processed by austenitation at different temperatures was examined by SEM, TEM and XRD. It was initially for structural transformation from austenization at different temperatures strongly affect the microstructure evolution during followed by ART-annealing, even the ultrafine grained ferrite / austenite duplex structure with about 30% austenite could be obtained after long time ART-annealing in all cases. Austenization in the intercritical region (between A c1 and A c3) gave a duplex结构 后 quenching, which was nearly not affected by followed annealing process. However, high temperature austenization (above A c3) resulted in a full martensite structure after quenching, which gradually transformed into a ferrite / austenite duplex structure during the following annealing process. on the analysis of austenite fraction and carbon concentrate, it was found that not only carbon partitioning but also manganese partitioning in the austenite affected the stability of austenite and even dominated the development of lamellar ferrite and austenite duplex structure during intercritical annealing with different times. At last an austenite lath nucleation and thickening model was proposed to describe the microstructure evolution of medium mangenese steel during ART-annealing.