Simulation of heat treatment processes presupposes that material data exist for the description of the phase transformation. During the heat treatment transformation and thermally caused stresses develop, which affect the behaviour of the phase transformation. The subject of this paper is the interaction between phase transformation and internal stress. The investigations show the influence of constant tensile loading on transformation strains occurring during the isothermal pearlitic and bainitic transformation and also the martensitic transformation of the bearing steel 100Cr6. The transformation plasticity parameter K increases during pearlitic transformation, with increasing transformation temperature linear, but decreasing for bainitic transformation with increasing temperature.