The stability of a new two-phase states with ellipsoidal nuclei is discussed. It is assumed that at the initial stage of phase transition of deformed material the new phase is formed in ellipsoidal nuclei. The interaction between the nuclei is taken into account in the approximation of the effective field approach. The dependence of the volume concentration, shape, and orientation of nuclei on the external field is determined by the free energy minimum condition. Zones of existence of equilibrium two-phase states, in the strain space, with ellipsoidal nuclei are constructed, as well as straining macrodiagram. It is necessary, for phase transformations to be possible, that the free energy density be a nonconvex function of strain represented by a set of quadratic functions. The phase transition is realized through the multiple appearance of ellipsoidal nuclei with characteristic functions, without ruling out the possibility of other two-phase structures.