Landau–Devonshire theory is used to investigate the effect of external mechanical stress (or strain) on domain structure and the remanent polarization of Pb(Zr0.35Ti0.65)O3 (PZT) thin films, by considering the competition between the external stress and the intrinsic stresses. A set of intrinsic stress functions in PZT film is obtained by solving elastic mechanical equations. At room temperature, the intrinsic stresses may lead to an alternate a/c/a/c domain structure. While an external in-plane tensile stress increases the Gibbs free energy of the c-phase and decreases that of a-phase. Parts of the c-domains turn to a-domains, so as to reduce the remanent polarization of the PZT films.