Formation of austenite strongly influences the microstructures and mechanical properties of dual phase steels. In present work, austenization process during intercritical annealing was studied in a Fe-C-Mn steel using Gleeble-1500 thermal simulator and quantitative microscopy. The experimental results show that austenite formation is separated into three different stages: (i) growth of high carbon austenite into pearlite rapidly until pearlite dissolution is completed; (ii) slower growth of austenite into ferrite; (iii) very slow equilibration between ferrite and austenite. The thermodynamic and kinetic analyses show that growth of austenite into ferrite is controlled by carbon diffusion in austenite in the primary stage and manganese diffusion in ferrite in the subsequent stage because diffusion coefficient of Mn in ferrite is several orders of magnitude smaller than that of C in austenite. The slow final equilibration between ferrite and austenite is obtained by manganese diffusion through the austenite. Based on the analysis, one dimensional diffusion model of intercritical austenization was developed and solved using finite volume method on the assumption that solute flux was local balance at interface, and the kinetics calculated was compared with experimental results. Simulated results indicate that growth of austenite reaches paraequilibrium in about one second, but remains thousands of seconds to reach final equilibrium. Simulated concentration profiles show that manganese atoms transferred from ferrite congregate in austenite near phase interface, which is consistent with the experimental phenomenon.