Self-assembled high-density gallium nitride (GaN) nanorod arrays have been grown on (0 0 0 1) Al2O3 substrates by metal-organic molecular beam epitaxy (MOMBE) using radio-frequency radical nitrogen. The corresponding microstructure and optical properties of nanorods were investigated by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), micro-Raman (mu-Raman) spectroscopy and temperature-dependent photoluminescence (PL). It was found that the length and diameter of nanorods varies with the growth temperatures, and the rod number density can reach around 10(10) cm(-2). HRTEM and corresponding diffraction patterns have revealed that the GaN nanorods have a dislocation-free, single-crystal hexagonal wurtzite structure. mu-Raman scattering reveals that the self-assembled nanorods are strain free in consistent with XRD and HRTEM results. In particular, the PL spectra show an abnormal behavior with increasing temperature. The so-called "S-shape" behavior, a red-blue-red shift of emission peak with increasing temperature, might result from the fluctuation in compositional contents or crystallographic defects. In contrast to previous works, the process requires neither catalyst nor the effect of nanometer-sized confinement such as carbon nanotubes. These strain-free GaN nanorods may find many important applications on the future development of optical nanodevices. (c) 2008 Elsevier B.V. All rights reserved.