An atomic force microscope (AFM) tip-induced direct nano-oxidation method was used to fabricate nanoscale heavily carbon-doped p-type GaAs oxide wires, where a p(++)-GaAs layer was grown by metalorganic molecular beam epitaxy (MOMBE). By improving the shape of an AFM tip by electron-beam-induced deposition of a-C using scanning electron microscope (SEM), and by adjusting the AFM oxidation process conditions, a p(++)-GaAs oxide wire with 10 nm width was successfully fabricated. From these results, it was clear that the sizes of p(++)-GaAs oxide wires could be controlled by adjusting the process conditions.