Effects of nitrogen and oxygen annealing on the carrier concentration, carrier mobility, electrical resistivity and PL characteristics as well as the crystallinity of ZnO films deposited on sapphire substrates by atomic layer deposition (ALD). X-ray diffraction (XRD), Scanning electron microscope (SEM), photoluminescence (PL) analyses, and Hall measurement were performed to investigate the crystallinity, optical properties and electrical properties of the ZnO thin films, respectively. According to the XRD analysis results the crystallinity of the ZnO film annealed in an oxygen atmosphere is better than that of the ZnO film annealed in a nitrogen atmosphere. Annealing undoped ZnO films grown by ALD at a high temperature above $600^{circ}C$ improves the crystallinity and enhances W emission but deteriorates the electrical conductivity of the flms. The resistivity of the ZnO film annealed particularly at $800^[circ}C$ in a nitrogen atmosphere is much higher than that annealed at the same temperature in an oxygen atmosphere.