ZnO films have been grown on C-plane sapphire substrate with the plasma-assisted metal-organic chemical vapor deposition (MOCVD) method. By using the X-ray diffraction (XRD) and calculation, it is found that there is tensile strain in the sample annealed for many times during the growing process, and compressive strain in the sample which is annealed only one time after growth. The photoluminescence spectra show that there is only one emitting peak at around 380nm for the sample annealed for many times and two peaks for the sample annealed for one time after growing.