We report on plasma-assisted molecular. beam epitaxy of ZnO on Al2O3 (0 0 0 1) substrates with an MgO buffer layer. We found a thin MgO buffer greatly influence the initial growth of ZnO by facilitating the adhesion and promoting the lateral epitaxy of ZnO, Consequently, a fl;it surface with a (3 x 3) reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Both in situ investigations and structural analysis revealed that the MgO buffer has a rocksalt structure and consists of a 2D wetting layer and 3D islands above. The change in surface energy by covering the substrate with an MgO wetting layer results in a drastic suppression of columnar growth and layer-by-layer epitaxy of ZnO is achieved. Thr resulting improvement in crystal quality of the ZnO epilayers is confirmed by a. large decrease in the X-ray rocking curve widths of both symmetrical and asymmetrical diffraction peaks, as well as the presence of free exciton recombinations in low temperature photoluminescence and the low ii-type background carrier concentration.