Influence of nitrogen and oxygen annealing atmospheres 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) were compared. X-ray diffraction (XRD) and photoluminescence (PL) analyses, and Hall measurement were performed to investigate the crystallinity, optical properties and electrical properties of the ZnO thin films, respectively. The UV emission intensity for oxygen annealing is stronger than that for nitrogen annealing in the case of annealing at 600°C, but the difference decreases with the Increase of annealing temperature. The strongest UV emission is obtained by oxygen annealing at 800°C. However, from the viewpoint of electrical resistivity annealing at 1,000°C in either an oxygen or a nitrogen atmosphere is more desirable. Taking both the PL and electrical properties into consideration it may be concluded that optimum annealing condition for ZnO thin films grown on the sapphire substrate by ALD is an annealing temperature of 900°C and an annealing atmosphere of oxygen although the effects of annealing atmosphere on the optical and electrical properties are not so significant.