Epitaxial needlelike ZnO nanowires were grown vertically over an entire epi-GaN/sapphire substrate at 550 degreesC by low-pressure vapor phase deposition without employing any metal catalysts. A two-step oxygen injection process is the key of successful synthesis. The length of ZnO wires was up to 3.0 mum. The diameters of the roots and tips of the ZnO nanowires were around 80-100 and 15-30 nm, respectively. X-ray diffraction showed the epitaxial orientation relationship between ZnO and GaN as [001](ZnO) //[001](GaN) along the normal to the plane, and [100](ZnO)//[100](GaN) along the in-plane direction, consistent with the selective area electron diffraction pattern taken at the ZnO/GaN heterointerface. High-resolution transmission electron microscopy confirmed that nanowire was a single crystal. A room-temperature photoluminescence spectrum of the wires revealed a low concentration of oxygen vacancy in the ZnO nanowires and showed high optical quality. (C) 2004 The Electrochemical Society.