In ZnO/sapphire (0001) heteroepitaxy, the properties of the ZnO thin films are ultimately affected by the biaxial strain induced by the lattice mismatch. In this work the strain relaxation process in the initial growth stage of ZnO/sapphire (001) by PLD and its effect on the film properties have been investigated. Detailed structural evolution of ZnO films was carried out using XRD and TEM. At low temperature and high O2 pressure, the strain is released at the very early stage of growth by roughening the film surface and forming 3D islands. TEM analysis showed a columnar growth mode, and the existence of two types of in-plane rotation domains, ZnO [10-10]//sapphire [10-10] and ZnO[11-20]// sapphire [10-10]. Interestingly, the films grown at high temperature and low O2 pressure exhibit (0002) rocking curves dominated by a sharp peak with FWHM of 0.02 degree, and a single in-plane orientation ZnO[11-20]// sapphire [10-10]. TEM investigation revealed also highly crystalline layers which contain only basal stacking faults. This improved crystalline quality at optimized growth condition is induced by a much slower strain relaxation, so that the ZnO epitaxial layer is aligned to the lattice of the underlying sapphire substrate.