A two-dimensional thermodynamic model was developed to account for the observed difficulty in the fabrication of epitaxialPb(Zr,Ti)O3(PZT) thin films in which tetragonal and rhombohedral phases coexist. The thermodynamic formalism based on the Landau–Devonshire’s phenomenological theory predicts the enhanced thermodynamic stability of the tetragonal-phase field under a two-dimensional compressive stress. We have experimentally proved this prediction by fabricating an epitaxially oriented tetragonal PZT thin film on MgO substrate with the target composition corresponding to the bulk morphotropic phase boundary (MPB). ©1998 American Institute of Physics.