Strain relaxation and its effect on domain formation of epitaxial PbTiO3 thin films grown on MgO (001) substrates were investigated as a function of film thickness by two-dimensional reciprocal space mapping using synchrotron x-ray diffraction. Within a few hundreds of angstrom region, it was observed that c-domain abundance, α, was critically dependent on film thickness. As the film thickness increased further, α was saturated at a value of ∼0.75. The HK mesh scan on PbTiO3 (100) reflections revealed that directional tilting of a domains with four-fold symmetry began to develop as the film thickness exceeded 650 Å. Thermodynamic equilibrium relief of the coherency strain was evaluated based on Mattews–Blakslee criteria that determine thickness dependent misfit accommodation. This theoretical consideration with experimental results led us to conclude that the unrelaxed residual misfit strain has a significant effect on the domain formation, particularly in the region below the thickness of 1000 Å. © 2000 American Institute of Physics.