Development of TRIP aided ferrous alloys is one important answer to the demand for weight decrease in the automotive industry. The microstructure of cold rolled TRIP steels is quite complex and the optimisation of such steel products requires a detailed understanding of the mechanisms of phase transformation, in the thermomechanical treatment as well as in the mechanical testing or metal forming. This paper presents the model developed recently to predict the microstructure and mechanical properties of TRIP steels. It is built on physical basis : thermodynamics and kinetics of nucleation and growth for transformation and precipitation, mechanical behavior obtained through the evolution of dislocation densities. A complete program describing the evolution of the microstructure all along the annealing process and the final mechanical properties was then designed. Comparisons with experimental data are finally presented and show satisfactory results.