Wurtzite-type aluminum nitride (w-AlN) films were epitaxially grown on sapphire substrates by electron-cyclotron-resonance plasma assisted metalorganic chemical vapor deposition (ECRMOCVD) using trimethylalminum (TMA) and ammonia (NH3 as source gases under a mirror field condition in a growth chamber. Under small gas feed ratios of N source to Al source (NH3/TMA) at epitaxial growth process, the proportion of non-epitaxial domains became large. In order to obtain AIN epitaxial films having excellent crystallinity and surface morphology on c-plane of sapphire substrates, thermal nitridation, the growth of buffer layer around 600 degreesC prior to the epitaxial growth, and the epitaxial growth under high NH(3/)TMA gas feed ratios were required.