A　tri-buffer　method　was　applied　to　achieve　layer-by-layer　growth　of　high-quality　ZnO　films　on　sapphire　(0001)　substrates　by　rf　plasma-assisted　molecular　beam　epitaxy　(MBE).　After　sufficient　nitridation　of　the　substrate,　MgO　and　ZnO　buffer　layers　were　subsequently　deposited　on　the　resulting　AIN　layer.　An　atomically　smooth　ZnO　surface　with　a　roughness　less　than　1　nm　in　a　10　mu　m　x　10　mu　m　scanned　area　was　obtained　with　this　method.　The　crystal　quality　was　also　improved,　as　characterized　by　reflection　high-energy　electron　diffraction　(RHEED),　x-ray　diffraction　(XRD),　Raman　spectroscopy,　and　transmission　electron　microscopy　(TEM).　The　results　indicate　that　the　tri-buffer　process　could　reduce　the　large　lattice　mismatch　between　ZnO　and　nitrided　sapphire　and　facilitate　the　two-dimensional　(2-D)　growth　of　the　ZnO　epilayer.　A　model　is　proposed　to　understand　the　observations.