By　a　metastable　rocksalt　ZnO　interfacial　structure　and　a　resulting　rocksalt-to-wurtzite　structure　transition　in　ZnO　buffer　layer,　polarity　control　of　Zn-polar　ZnO　films　was　realized　on　rocksalt　MgO　(1　1　1)　template　by　plasma-assisted　molecular　beam　epitaxy.　The　metastable　rocksalt　ZnO　and　the　phase　transition　from　octahedrally　bonded　rocksalt　to　tetrahedrally　bonded　wurtzite　structure　during　the　growth　of　ZnO　buffer　layer　were　established　by　the　combination　of　in　situ　reflection　high-energy　electron　diffraction　and　ex　situ　high-resolution　transmission　electron　microscopy.　Such　a　metastable　rocksalt　ZnO　has　a　lattice　constant　a　=　4.16　angstrom　and　a　typical　...abcabc...　atom　stacking　along　the　growth　direction.　Polarity　selection　of　Zn-polar　ZnO　films　was　found　greatly　depending　on　the　bonding　configuration　of　the　epitaxial　ZnO　interfacial　layer,　and　an　atomic　model　of　the　structure　transition　was　proposed　to　explain　the　mechanism　of　ZnO　polarity　selection.　(C)　2009　Elsevier　B.V.　All　rights　reserved.