Stress relaxation in PbTiO3 films was investigated by the in situ stress measurement technique. A simple viscous flow model was successfully used to interpret the kinetics and behavior of stress relaxation of PbTiO3 thin films. The activation energy responsible for stress relaxation was estimated to be 190 kJ/mole, which was accounted for by the lattice diffusion of vacancies. A Nabarro-Herring creep model was successfully employed to correlate the relationships among the viscosity, lattice diffusion coefficient, and grain size of the PbTiO3 films, and an estimate of the lattice diffusion coefficient of vacancy motion during relaxation was obtained. Also, the observed time required for complete relaxation was found to be in accord with theoretical values. Hillock formation resulting from grain boundary sliding is believed to contribute to stress relaxation in its early stage. Thereafter, grain growth resulting from lattice diffusion is believed to play a major role in the stress relaxation.