Submicron-size wires of Co-Al-O granular film were fabricated using a gallium focused ion beam, and the magnetoresistance (MR) of the wires was investigated. Bias voltage dependence of the MR for Co-Al-O films was measured up to 60 mV utilizing a wire geometry with cross-sectional area of 100×100 nm² and length of 500 nm. MR is constant in the range of V_b = 0 to 60 mV at 77 K, where V_b is defined as the bias voltage between two neighboring Co granules in the wire, but the MR magnitude is slightly smaller than that before fabrication by the focused ion beam. The decrease in MR is thought to be caused by an irradiation effect of high-energy gallium ions.