High density vertical single crystal ZnO nanowires were selectively grown on ZnO:Ga/Si3N4/SiO2/Si templates at various temperatures by a two-step oxygen injection process of self-catalyzed vapor-liquid-solid (VLS) technology. It was found that tips of the ZnO nanowires are hexagonal. It was also found that average length of the ZnO nanowires increased while the average tip diameter of the ZnO nanowires decreased as the growth temperature increased. Furthermore, it was found that the ZnO nanowires grown at 500 degrees C were '' tube-shaped '' while the ZnO nanowires grown at 700 degrees C were '' cone-shaped.'' Photoluminescence (PL). x-ray diffraction (XRD), and energy depersive x-ray (EDX) results all indicate that the quality of our ZnO nanowires is good. (c) 2005 American Vacuum Society.