The polarity of ZnO films grown on MgO/c-Al2O3 templates are successfully controlled by only varying MgO buffer layer thickness. The crystal structure of the MgO buffer on c-Al2O3 is Of wurtzite (WZ), when the MgO buffer thickness is less than 2.7 nm. As the MgO layer thickness exceeds, the crystal structure of MgO changes to rock salt (RS) structure. A ZnO layer grown on WZ-MgO results in O-polar ZnO, while the polarity of the ZnO layer grown on RS-MgO changes to Zn polar. Possible atomic configurations of the ZnO/MgO/Al2O3 interface structure are suggested. (c) 2005 American Vacuum Society.