GaN nanowires were successfully synthesized on Si(111) substrates by ammoniating the Ga2O3/ZnO films at 900 °C. The structure, morphology and optical property of the as-prepared GaN nanowires were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), field-emission transmission electron microscope (FETEM), Fourier transform infrared spectrum (FTIR) and fluorescence spectrophotometer. The results show that the GaN nanowires have a hexagonal wurtzite structure with lengths of about several micrometers and diameters ranging from 30 nm to 120 nm. The representative photoluminescence spectrum at room temperature exhibited a strong emission peak at 374.1 nm and two weak emission peaks at 437.4 nm and 473.3 nm. Finally, the growth mechanism is also briefly discussed.