Studies on the polycrystalline PbTiO3 thin films deposited on (001) redoped n‐Si substrates by metalorganic chemical vapor deposition (MOCVD) under reduced pressure indicated the evidence of ferroelectricity weakening [Appl. Phys. Lett. 65, 1906 (1994)] when the grain size of PbTiO3 was down to the scale of ∼1100 Å. X‐ray diffraction patterns demonstrated that the perovskite unit cells have a contraction along the c‐axis direction. All transverse optical phonon modes in the films were observed shift downward in the Raman spectrum measured at 300 K. And the spontaneous polarization of PbTiO3 is determined to be 52 μC/cm2 by using Si substrates as the bottom electrodes. The three related experimental observations are conjected to be attributable to the finite size effect and the surface conditions in the ferroelectric thin film.