ZnO is an n-type semiconductor having a hexagonal wurtzite structure. By X-ray diffraction (XRD) and scanning electron microscope (SEM), the influences of substrate temperature, the ratio of Ar to O2 and thermal temperature on ZnO crystal quality were studied. The results show that ZnO films deaposited at substrate temperature of 240 Celsius degrees and Ar:O2=1:3 have the best crystallization. UV photoluminescence is observed when ZnO films are excited by He-Cd laser at room temperature. Stress at boundary causes an intrinsic UV emission peak shift to the lower energy. Oxygen vacancy or zinc interstitial causes deep-level emission. With higher substrate temperature, the crystallization is improved and the stress and deep-level green emission are reduced.