Crystal structure change with the temperature was investigated for 3 m-thick (100)/(001)-oriented epitaxial PbTiO3 films grown on SrTiO3 substrates. Complex strain-relaxed domain structure labeled as Type III was observed and directly transformed to the cubic phase at about 490°C. This transition temperature and the lattice parameter (a and c- axes) change with the temperature well agreed with the reported data for the PbTiO3 powders. The volume fraction of the (001) orientations, Vc, was almost independent of the temperature up to the phase transition temperature. The tilting angles of the spots in XRD plan view were almost the same with the estimated ones from the lattice parameters and the Vc. This suggests that the angle of the domains identified by the domain structure in Type III. This structure is mainly determined by the tetragonality, (c/a ratio) and the Vc.