We observed the morphologies of GaN surfaces before and after ion sputtering by atomic force microscopy (AFM). The GaN surfaces were bombarded by an O-2(+)-beam in an SIMS apparatus with different bombarding energies, incident angles, ion current densities, and ion doses. The morphologies of the ion-bombarded surfaces were largely different at various incident angles. The roughness was largest around 45degrees at any bombarding energy. The roughness did not monotonously become larger by increasing the bombarding energy but was the smallest at 5 keV Both of the ripple height and wavelength were found to be almost unchanged even though the ion current density was varied. The ripple height became much higher by increasing the ion dose, whereas the wavelength became larger with only a small increment. (C) 2002 Elsevier Science B.V. All rights reserved.