The adsorption of molecular oxygen on GaN (0001) and (0001) surfaces has been explored by employing density functional theory. Our calculations indicate that O-2 prefers to be dissociated on these surfaces in parallel orientation, and the dissociated O atoms are combined in fcc site (on both surfaces) or in hcp site (only on GaN (000 (1) over bar) surface). The lengths of the formed Ga-O bonds at the two polar surfaces agree with that in ideal Ga2O3 bulk, and these reactions are greatly exothermic. From the potential curve, no obvious energy barrier is found during the process of O-2 dissociated adsorption on GaN (0001) surface. (c) 2006 Elsevier B.V. All rights reserved.