The ozonization of sapphire(0 0 0 1) substrates has been performed prior to metalorganic vapor phase epitaxy (MOVPE) to remove carbon-pollution of sapphire substrates. The ozonization was carried out using oxygen gas and low-pressure mercury lamp irradiation. The result of X-ray diffraction indicates that the distribution of c-axis tilt in the GaN layer was reduced by ozonization treatment. The photoluminescence intensity was higher than that without ozonization. The surface morphology of GaN layers also became smooth. The ozonization process is found to be effective for growing a high-quality GaN on sapphire.