Metalorganic chemical vapor deposition (MOCVD) growth of AlN on sapphire substrates was investigated to realize pit-free smooth surfaces. Growth conditions were optimized using two-step growth technique, in which the first-step growth was done at a low temperature (1200 degreesC) and followed by the second-step growth at a high temperature. At the first-step growth. the substrate was entirely covered by two-dimensionally grown AlN by decreasing V/III ratio to 1.5, although microcrystalline islands were observed at V/III ratios of 1.2 and 4.0. An almost pit-free flat surface was obtained after the second-step growth at 1270 degreesC. Further improvement was expected to be realized by raising the second-step growth temperature. Premature vapor-phase reaction should be suppressed to utilize the high-temperature growth.