Atmospheric pressure metalorganic vapor phase epitaxial growth and characterization of high quality GaN on sapphire (0001) substrates are reported. Using AlN buffer layers, GaN thin films with optically flat surfaces free from cracks are successfully grown. The narrowest x-ray rocking curve from the (0006) plane is 2.70' and from the (202̄4) plane is 1.86'. Photoluminescence spectra show strong near band edge emission. The growth condition dependence of crystalline quality is also studied.