Oxygen radicals pregrowth treatment and surface nitridation were used to eliminate Zn-polar inversion domains and control the growth of single-domain O-polar ZnO film on sapphire (0001) substrate by rf plasma-assisted molecular beam epitaxy. We found that the formation of oxygen-terminated sapphire surface prior to nitridation is crucial for achieving the anion polarity in subsequent AlN and ZnO layers, as demonstrated by formation of the 3×3 surface reconstruction during ZnO growth and ex situ polarity determination. This method, in general, can be applied to growth of other polar films, such as II-VI oxides and III-V nitrides, on sapphire (0001) substrates