The undoped ZnO films were grown on silicon (001) substrates by radio frequency magnetron sputtering. The dependence of defect formation and photoluminescence (PL) of ZnO films on the annealing temperature and oxygen mole ratio (OMR) were investigated using X-ray diffraction and PL spectra. A sharp ZnO (002) peak with a strong UV emission peak around 3.28 eV can be obtained for the films annealed in O2 and N2 atmospheres. However, the films annealed in nitrogen show strong deep-level emission peaks that vary with the annealing temperature. Below 850°C, Zn interstitials become the dominant point defects, but for the ZnO films annealed at higher temperatures such as 1000°C, oxygen vacancies become the predominant point defects. In contrast, in an oxygen atmosphere, a strong UV emission along with invisible deep-level peaks can be detected for ZnO films sputtered at an OMR of 5% and annealed at 850°C. This result is attributed to the enhanced crystallization and a reduced defect concentration.