GaAs nanowires (NWs) were generated on the surface of GaAs(111)B and GaAs(100) substrates from molecular fluxes by the self-catalytic growth method. A mask for NW growth was fabricated by oxidizing the epitaxial silicon layer that was grown on a substrate surface by the molecular beam epitaxy (MBE) method. Silicon was oxidized in purified air without moving the structures out of the vacuum system of the MBE apparatus. The process of Si/GaAs heterostructure oxidation was investigated using single-wave and spectral ellipsometry. The oxidized silicon surface morphology was studied by the atomic force microscopy methods. The scanning electronic microscopy method was used to examine the samples with NWs. The NW density was about 2.6 × 107 and 3 × 107 cm–2 for (111)B and (100), respectively.