Structural and optical properties of as-synthesized, Ga2O3-coated, and Al2O3-coated GaN nanowires are examined in this paper. GaN nanowires were synthesized by thermal evaporation of ball-milled GaN powders in an NH3 atmosphere. The thermal annealing of the as-synthesized GaN nanowires in an argon atmosphere allows their surfaces to be oxidized, leading to the formation of 2 nm-thick Ga2O3 layers. For the oxidized GaN nanowires, the distances between the neighboring lattice planes are shortened, and an excitonic emission band is remarkably enhanced in intensity, compared with the as-synthesized GaN nanowires. In addition, the as-synthesized GaN nanowires were coated cylindrically with Al2O3 by atomic layer deposition technique. Our study suggests that the Al2O3-coating passivates some of surface states in the GaN nanowires. (C) 2004 Elsevier B.V. All rights reserved.