Ferroelectric thin films have received significant attention in view of their applications in microelectromechanical systems. Correlations between microstructure and polarization behavior have not been established. Polycrystalline (Pb0.94La0.04)(Zr0.60Ti0.40)O3 thin films with thicknesses ranging from 100 to 400 nm were prepared on the Pt(111)/Ti/SiO2/Si(100) substrates by a sol–gel method. Synchrotron X-ray diffraction results revealed that the films thinner than 150 nm possess an asymmetry-allowed monoclinic (MA) phase and undergo a MA–monoclinic MB–R phase sequence with increasing thickness. It was found that longitudinal piezoelectric coefficient d33 optimizes at 128 pm/V for 150 nm thick film. Above this thickness, a simple decreasing trend was observed. The presence of the monoclinic phase and (100) preferred orientation as well as crystalline quality are considered to play key roles in enhancing piezoelectric properties of the 150 nm thick film. Piezoelectric materials show a composition-related transition region where the crystal structure changes abruptly and the piezoelectric properties are maximal. In this study, an alternative approach is proposed in achieving excellent piezoelectric properties via a thickness-dependent constrain effect.