A　Ga　wetting　layer　was　used　to　modify　the　surface　structure　of　sapphire　(0001)　substrate　to　prepare　high-quality　ZnO　film　by　radio　frequency　plasma-assisted　molecule　beam　epitaxy.　We　found　that　this　Ga　layer　plays　a　crucial　role　in　eliminating　30degrees　rotation　domains,　controlling　polarity　and　decreasing　defect　density　in　ZnO　epilayers,　as　demonstrated　by　in　situ　reflection　high　energy　electron　diffraction,　ex　situ　high　resolution　X-ray　diffraction　and　high　resolution　cross-sectional　transmission　electron　microscopy.　Zn-polar　film　of　ZnO　was　determined　by　convergent　beam　electron　diffraction.　A　Ga　bilayer　model　is　proposed　to　understand　the　effects　of　the　Ga　wetting　layer　on　high-quality　ZnO　growth.