The effect of oxygen pressure on the synthesis of ZnO nanowires by means of melt-oxidation of an Al-Zn mixture was investigated. The samples were prepared in oxygen ambient for 1 h at $1,000^{circ}C$ under oxygen pressure ranging from 0.5 to 100 Torr. ZnO nanowires were formed at oxygen pressures lower than 10 Torr. As the oxygen pressure increased from 0.5 to 10 Torr, the width of the nanowires increased, but their length decreased. The ZnO nanowires had a needle shape, which became gradually thinner toward the tip. X-ray diffraction patterns showed that the nanowires had a hexagonal wurtzite structure. However, ZnO nanowires were not observed when the oxygen pressure increased from 10 Torr to 100 Torr. In roomtemperature cathodeluminescence spectra of the ZnO nanowires, the intensity of ultra-violet emission at 380 nm increased with decreasing oxygen pressure, which indicated that the lower the oxygen pressure, the better the crystallinity of the ZnO nanowires.