The formation mechanism of experimentally observed epitaxial morphologies on ( 000 +/- 1) polar surfaces of ZnO microwire was studied by density functional theory simulation of atomic step models on both polar surfaces. In situ observations by environmental scanning electron microscopy were employed to control and detect the morphology evolutions during the epitaxial growth process. Edge formation energies for representative step models were calculated as a function of Zn chemical potential. The energetically favorable step structures were determined under different conditions, which could be related to certain surface morphologies regarding the aspects of crystallography and growth kinetics. Our experiments can be well explained by theoretical simulations.