Plasma-assisted molecular beam epitaxy (P-MBE) of ZnO films on Al2O3(0 0 0 1) substrates is reported. By introducing a thin buffer layer of MgO, surface morphology can be greatly improved at the initial growth stage, which leads to an atomically flat ZnO surface with (3 x 3) reconstruction. During the consequent ZnO film deposition, RHEED intensity oscillations longer than 50 periods can be observed, which evidences the two-dimensional epitaxial growth. Morphological, structural, and optical investigations indicate that by using the MgO buffer layer a great improvement, of surface flatness and crystal quality is achieved for the ZnO films grown on Al2O3(0 0 0 1) substrates by P-MBE.