This paper reports the effects of a GaN buffer layer in the OMVPE growth of InN on sapphire substrates. The GaN buffer is grown at 550degreesC and then annealed at different temperatures. The effects of GaN buffer are found to be dependent on the position of the substrate on the 18 cm-long susceptor. Such a dependence on substrate position is caused by a different effective V/III ratio; a lower V/III ratio near the upstream end and a higher V/III ratio near the downstream end. Uniformity of morphology for grown InN film is markedly improved by employing a GaN buffer layer. This improvement is due to the uniform nucleation of InN. The crystalline quality of the GaN buffer is improved with increasing annealing temperature. However, the crystalline quality of an InN film is almost insensitive to that of the underlying layer when the film is grown under the high effective V/III ratio condition. By employing a GaN buffer, electrical property of InN is improved; a carrier concentration of 1.1 x 10(19) cm(-3) and a Hall mobility of 870 cm(2) V-1 s(-1) are obtained as the best data. It is believed that the mobility of 870 cm(2) V-1 s(-1) is the highest ever reported for OMVPE InN. (C) 2003 Elsevier B.V. All rights reserved.