Glomerate GaN nanowires were synthesized on Si(111) substrates by annealing sputtered Ga(2)O(3)/Co films under flowing ammonia at temperature of 950 degrees C. X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy and Fourier transformed infrared spectra were used to characterize the morphology, crystallinity and microstructure of the as-synthesized samples. Our results show that the samples are of hexagonal wurtzite structure. For the majority of GaN nanowires, the length is up to tens of microns and the diameter is in the range of 50-200 nm. The growth process of the GaN nanowires is dominated by Co-Ga-N alloy mechanism.