MOVPE growth of GaN on both polar (0001) planes of 6H-SiC and on (0001) sapphire is described. The surface morphology and the photoluminescence property of the epitaxial layer are found to depend largely on the substrate polarity. A previously proposed XPS method was extended to examine the GaN surface. As a result, the GaN polarity has been determined for the first time. The polarity of GaN/sapphire coincides with our previously reported inference from Zn-incorporation efficiency. In conclusion, the GaN polarity changes in accordance with the substrate polarity.