A high density of tungsten oxynitride nanowires was synthesized by ammonia annealing of WO3 nanowires, which were grown on a tungsten substrate by thermal evaporation of WO3 powder. The morphology of WO3 nanowires was not changed upon ammonia annealing at 650 C, although the color of the sample changed drastically. X-ray diffraction and transmission electron microscopy analyses showed that the structure of the ammonia annealed nanowires matched well, with a cubic oxynitride, with the structure of W-0.62(N0.62O0.38). The x-ray photoelectron spectroscopy and x-ray spectroscopy also confirmed the formation of the tungsten oxynitride nanowires. Field emission measurements showed a low turn-on field of 4.45 V/mu m for the WNxOy nanowires, indicating that they can be used as potential field emitters. (C) 2009 American Vacuum Society.