First principles density functional theory was used to determine the ZnO preferred orientation. We report that the (21¯1¯0)-oriented ZnO surface is more stable than others. ZnO will changes its preferred orientation from (21¯1¯0) to O-terminated (0001) under O-rich conditions. In addition, our results show that the O-terminated ZnO (0001) polar surface is the preferred termination of the ZnO (0001) surface. Moreover, the [112¯l] direction of ZnO films is the epitaxial strain stabilized orientation for initiating ZnO {112¯l} planes.