Morphology evolution of ZnO(000 1¯) surface during plasma-assisted molecular-beam epitaxy was studied. In a step-flow growth mode, terraces bounded by anisotropic steps along <11 2¯0> characterize the surface of ZnO epilayers. Adatoms favorably incorporate at upsteps, which translates hexagonal islands to regular terraces with equalized width. Surface morphology is sensitive to the Zn/O ratio. Oxygen stabilizes mobile Zn adatoms hence oxygen rich conditions result in rough step edges and irregular hexagonal terraces. Steps become smoother with increasing Zn/O ratio until the stoichiometric condition is achieved. As the Zn flux exceeds the stoichiometry, faceted hexagonal pits form on the ZnO(000 1¯) surface. Rather than the lack of surface mobility, the lack of stabilization of adatoms is responsible to the formation of pit. © 2002 American Institute of Physics.