Structural characteristics of the ZnO epitaxial films grown on c-plane sapphire by atomic layer deposition method were thoroughly studied. The morphology of thin film is smooth. The in-plane axes of the c-plane oriented ZnO layers are predominantly aligned with that of the sapphire substrate, yielding the relationship of . The minor orientation with a 30° in-plane twist configuration, i.e. , which is more commonly observed in ZnO films grown by metal organic chemical vapor deposition, pulsed laser deposition and other methods, only amounts to less than 3% and can be eliminated by thermal annealing. The structure of the ZnO epi-films exhibits significantly improvement upon thermal annealing and intrinsic types of basal plan stacking faults are the predominant structural defects in the ZnO after thermal treatment.