One-dimensional GaN nanostructure films were successfully synthesized by the recently developed sputtering post-nitridation technique. The morphology and structure of GaN nanowires are investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The results indicate that the crystalline GaN nanostructures have a hexagonal wurtzite structure, and there is not any other phase such as Ga2O3 or Ga in the specimen. It also confirms that high quality crystal was obtained in the resulting sample due to the lattice defects decreased and the crystallinity improved in the process of nitridation at high temperature. The growth mechanism of the GaN nanostructures is briefly discussed.