GaN nanowires were successfully grown on Ni-coated Si substrate by direct reaction of gallium with ammonia gas in a home-made vertical tubular chemical vapor deposition reactor. The growth of GaN nanowires was uniformly observed across the Si substrate surface, but the density and average diameter of the nanowires varied along the position of the substrate surface. At the position of 5 cm above Ga source surface, the growth of GaN crystal grains was observed with few nanowires. The length of the nanowires reached several micrometers. The clear lattice fringes in HRTEM image revealed the growth of good quality hexagonal single-crystal GaN nanowires. Photoluminescence of the GaN nanowires showed a strong band edge emission at the energy position of ~3.4 eV with negligible deep level yellow emission. Field emission characteristics of the GaN nanowires showed that the turn-on field of GaN nanowires was ~7.4 V/um with a field enhancing factor β of ~555. The catalytic growth mechanism of the GaN nanowires was discussed on the basis of experimental results in this work.