Twin formation and strain evolution in highly mismatched sputter-grown ZnO/Al2O3(0001) hereroepitaxial films were investigated using real time synchrotron x-ray scattering measurements and high resolution electron microscopy (HREM). We reveal the existence of a critical thickness at which the twin of the 30degrees rotated domains starts to nucleate within the ZnO films, followed by gradual strain relaxation. Twin growth above the critical thickness stops as the strain almost fully relaxes. A HREM image shows that the twin nucleates 5-10 nm away from the interface as isolated domains, not as types of layers. (C) 2004 American Vacuum Society.