We report growth condition optimizations of ZnO films by plasma-assisted molecular beam epitaxy. Effects of oxygen plasma conditions on film quality were evaluated by photoluminescence, X-ray characterization and scanning electron microscopy. From the growth experiments on c-plane sapphire and bulk ZnO ( + c, - c) substrates, it was suggested that the surface morphology is strongly related with the growth direction of ZnO parallel to its c-axis, and was found to be controllable by optimization of oxygen plasma conditions.