A focused Ga+ ion-beam (FIB) writing system has been used to directly change an insulating platinum oxide film into a conducting film with a dose in the order of 1014 Ga+/cm2 at 30 keV ion energy. The sheet resistance of a PtO2 film, which is prepared by magnetron sputtering, was reduced from 4×109 Ω/□ to approximately 5×102 Ω/□. Electron microprobe measurements indicate that oxygen loss in the irradiated regions causes the large decrease in resistivity. Scanning electron microscope pictures show that the film quality after ion irradiation is more homogeneous than after laser irradiation, which has been used to pattern these materials by thermal processes. Compared with laser patterning, the resolution of FIB patterning is more than one order of magnitude higher and is suitable for possible applications in nanotechnology. © 2000 American Institute of Physics.