The in-plane polarities of GaN and ZnO non-polar films deposited on r- and m-sapphire are compared. The polarity is unique on r-sapphire and mixed on m-sapphire because the direction on the substrate surface parallel to the wurztite c-direction is polar in the first case and non-polar in the second case. Furthermore, on r-sapphire where GaN and ZnO have a unique polarity, the directions of the polar c-axis relative to a reference sense in the substrate surface are opposite for both materials. This difference may be related to the nitridation which is the first step of the of III-nitride growth. (C) 2009 The Japan Society of Applied Physics