Large-area piezoelectric ZnO films with different gran size has been synthesized by sol-gel technique using different annealing temperatures from 550 to 700°C. The piezoelectric efficiency (PE) of those deposited films is characterized by Piezoelectric Force Microscopy (PFM). All synthesized films exhibit a crystal structure. The width of the rock curve of [0002] characterized by x-ray diffraction decreases with the annealing temperature, suggesting a better c-axis orientated ZnO film formed at higher annealing temperature. The grain size of the grown films are found to continuously increase from 20 to 60 nm when the annealing temperatures increase from 550 to 700°C. The piezoelectric efficiency (PE, d 33 ) of the films exhibit strong grain size dependence, i.e., the PE initially increase with the annealing temperature and then decreasing with a further annealing temperature increased. The maximum PE value appears in the film annealed at 650°C. The peculiar piezoelectric properties (d 33 ) can be explained by the competing between the crystalline, which favors a larger d 33 due to the enhanced dipole polarization, and the grain size, which results in a piezoforce release at large grain size due to domain wall size and motion.