GaN nanowires were successfully grown on a Ni-coated Si substrate with a direct reaction of gallium and ammonia in a home-made vertical tubular chemical vapor deposition (CVD) reactor. The grown GaN nanowires were uniformly distributed across the Si substrate surface, but the density and diameter of the nanowires varied along the length of the substrate surface. The clear lattice fringes in the high-resolution transmission electron microscopy (HRTEM) image indicated the growth of good quality hexagonal single-crystal GaN nanowires. The GaN nanowires produced a strong band edge emission at similar to 3.4 eV with a negligible deep level yellow emission. Field emission characteristics of the GaN nanowires showed that the turn-on field was similar to 7.4 V/mum with a field enhancing factor beta of similar to 555.