The lattice constants of tetragonal PbTiO3grown by organometallic chemical vapor deposition, vary with average crystallite size in the thin film as determined from line broadening in x‐ray diffraction patterns. Thea‐ andc‐lattice constants become equal when the average crystallite size decreases to below ∼5 nm. These data agree with the change in lattice constants as a function of average crystallite size found recently for submicron PbTiO3powder.