An optimal simulation of heat treatment processes presupposes that the materials data to describe the phase transformation are available. During the heat treatment the phase transformation and thermal expansion causes stresses, which influence the transformation behaviour. The interaction between phase transformation and internal stresses was the aim of this investigation. For isothermic pearlitic and bainitic transformation of the bearing steel 100Cr6 the dependence of the transformation kinetics and plasticity on the temperature, the transforming phase, and on the stress state was studied. It was observed that an acceleration of the phase transformation takes place by external stresses only in limited temperature ranges. With increasing transformation temperature the material parameter K, which marks the transformation plasticity, increases linear for pearlitic transformation and decreases for bainitic transformation.