We have synthesized GaN nanocrystals by sequential implantation of Ga and N ions into a sapphire substrate followed by a postimplantation anneal. The nanocrystals have been identified as the wurtzite phase &agr;-GaN structure by transmission electron microscopy. We also found that the nanocrystals are aligned with the sapphire following the relationship:(0001)sapphire∥(0001)GaNand(112¯0)sapphire∥(112¯0)GaN.The use of a sapphire substrate allows for the measurements of optical properties, and near band-edge luminescence and the yellow band are observed in photoluminescence spectroscopy.