The dependence of the transformation rate on the multiaxial stress state corresponding to different loading paths is investigated. On the basis of the single shear, uniaxial tension, plane strain and equibiaxial stretching tests, the influence of stress state on stability of retained austenite is analyzed and an equation of transformation kinetics is developed. In sheet metal forming, the material undergoes complicated deformation, the prediction of the volume fraction of retained austenite during forming process is essential to estimate the contribution of the TRIP effect to improving formability. To this aim, the volume fraction of retained austenite in the part’s different regions has been calculated using finite element method. The calculated results were compared with the experimental data measured in deep drawing experiments.