GaAs quantum wires of a new type are fabricated on {1¯10} crystallographic facets perpendicular to the (1¯1¯1¯)Bsubstrates by selective area growth using metalorganic chemical vapor deposition. First, rectangular‐shaped AlGaAs layers are grown on a SiO2stripe‐masked GaAs (1¯1¯1¯)Bsubstrate at a high growth temperature. Next,n‐AlGaAs/GaAs modulation‐doped structures are laterally grown on {1¯10} sidewalls at a low growth temperature. The channel width of the one‐dimensional electron gas can be exactly controlled by the thickness of the first rectangular AlGaAs layer. The existence of the quasi‐one‐dimensional electron gas on {1¯10} sidewalls is confirmed by the orientation dependence of the Shubnikov‐de Hass oscillations. The advantage of this quantum wire structure is that there is no size fluctuation which is usually present when dry etching processes are used. Other applications of this selective growth on masked substrates, such as lateral superlattices, are also discussed.