The top-down fabrication of an in-plane nanowire NW GaAs metal-oxide-semiconductor field-effect transistor MOSFET by focused-ion beam FIB etching and chemical oxidation is reported. The device has a semiconductor-on-insulator structure with an n -GaAsAl2O3 layer stack implemented by lateral hydrolyzation oxidation. A 2 micron-long channel having an effective cross section 70 x 220 nm is directly fabricated into the n -GaAs layer by FIB etching. The channel is electronically isolated from the substrate by the Al2O3 layer and is effectively an in-plane NW epitaxially connected to the source and drain for a planar MOSFET within a single layer. The NW channel is surrounded by an 15 nm-thick gate oxide.