Recently, the wide bandgap semiconductor GaN and its alloys have been interested because of their superb properties for the fabrication of blue- or ultraviolet-light emitting devices. There is much interest in growth mechanism and the reduction of defects. The reported crystallographic structure of the GaN film is usually wurtzite or zinc blend type. Because the most common growth direction of the GaN film is normal to the {0001} plane of the wurtzite structure, it is very important to determine the polarity of the film. We determined the polarity of the GaN thin films grown on sapphire substrates by using anomalous X-ray diffraction techniques.