The deformation during phase transformation and phase transformation under stress are phenomena at the base of many important effects such as superelasticity, pseudoplastisticity and shape memory. In this paper the authors consider the thermodynamics of coherent phase transformation under a given variable strain. The theoretical background of the paper is the thermodynamical theory of heterophase and polydomain microstructure and their deformation. They use a realistic model that, however, is simple in form and allows one to obtain an exact solution of the problem.