Using x-ray reflectivity and high resolution x-ray diffraction, I report the structural studies of ZnO thin films which were epitaxially grown on sapphire from few to thousands atomic layers by atomic layer deposition (ALD). As well known, x-ray reflectivity and diffraction measurements are ideal for probing the structural information of films, such as the thickness, roughness and densities of layers. In this study, I measured the surface and interface morphological characteristics as a function of thickness of ZnO films. I also observed the existence of a diffusion layer between the substrate and ZnO film in all the studied films. From the phi-scan, all the samples shows the hexagonal symmetry.