Single-crystalline GaN nanowires have been synthesized on Si(111) substrates by magnetron sputtering through ammoniating the Ga2O3/Nb films at 900 °C in a quartz tube. The as-prepared nanowires are confirmed as single crystalline GaN with wurtzite structure by X-ray diffraction (XRD), selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). Transmission electron microscopy (TEM) shows that the GaN nanowires are straight and smooth, and possess the diameters of about 50 nm and lengths up to several microns. When excited by 325 nm helium-cadmium (He-Cd) laser light at room temperature, the GaN nanowires only have a strong ultraviolet luminescence peak located at 367 nm, owing to GaN band-edge emission. Finally, the growth mechanism of GaN nanowires is discussed briefly.