The phase diagram of a PbZr1−xTixO3 (PZT) film constrained by a much thicker substrate was studied using both thermodynamic calculations and phase-field approach. It was found that the ferroelectric transition temperature is increased with substrate constraint regardless of the nature of the constraint, i.e., tensile or compressive. The maximum increase in the transition temperature occurs near x = 0.5, and the morphotropic phase boundary is shifted considerably by the substrate constraint. It is shown that the orthorhombic phase (∣P1∣ = ∣P2∣ ≠ 0, P3 = 0) that does not exist in the bulk becomes stable under a tensile constraint, and the rhombohedral phase (∣P1∣ = ∣P2∣ = ∣P3∣ ≠ 0) in the bulk is distorted in the constrained film, i.e., ∣P1∣ = ∣P2∣ ≠ 0, ∣P3∣ ≠ 0. The phase diagrams obtained by the phase-field approach indicated that the stability region for the tetragonal phase is much wider than that obtained from the thermodynamic calculations assuming a single-domain, especially under tensile substrate constraint. The discrepancy between the two methods becomes larger as substrate constraint changes from compressive to tensile, implying that thermodynamic calculations are unreliable for constructing stability diagram of PZT, particularly under a tensile constraint. © 2003 American Institute of Physics.