A model using an energy balance is proposed to describe the volume fraction of multiple-interface martensitic transformations. For martensitic transformations without external stresses at quenching temperature T Ͻ M s , the volume fraction of martensite () is proportional to the undercooling (M s Ϫ T) and inversely proportional to a linear function of the quenching temperature (T); thus, ϭwhere  is a material constant. For stress-induced martensitic transformations under stress with temperature T Ͼ M s , the relationship is ϭ 0 [1 Ϫ ( Ϫ a a ik ik ik )] Ϫ1 , where 0 is the initial detectable amount of martensite formed at martensitic starting stress Ms ik and is a material constant. It is found that the results obtained from this model are in good M s ik ik agreement with experimental results.