Two-phase deformation of an elastic body is studied by a small strain approach. Phase transformations of martensite type are implied. An arbitrary anisotropy of the phases and arbitrary transformation eigenstrains are taken into account. An additional thermodynamic condition on the equilibrium interface and a configurational force are presented in a form relating an effective strain or stress on one side of the interface and the normal to the interface. The characteristic features and the shape of the equilibrium ellipsoidal and cylindrical new phase inclusions are studied. Then the difference between the equilibrium and optimal ellipsoidal inclusions is discussed. A necessary stability conditions is proved. An expression of the configurational force acting on the interface of an ellipsoidal inclusion is derived in a form convenient for the analysis. An example of the analytical examination of the behavior of the inclusion not far from the equilibrium is given.