The first demonstration ofinsituelectron‐beam (EB) lithography is reported, where a photo‐oxidized surface thin layer of GaAs is used for a resist. TheinsituEB lithography sequence consists of five processes, i.e., preparation of a clean GaAs surface, photo‐oxidation for a resist film formation, direct patterning of the oxide resist by EB‐induced Cl2etching, Cl2gas etching of GaAs surface for pattern transfer, and thermal treatment in an arsenic ambient for resist removal and surface cleaning. The GaAs wafer is never exposed to air throughout all of the above processes to avoid an unintentional surface contamination. The minimum electron dose required for patterning of the GaAs oxide resist is about 5×1016cm−2. An overgrown layer on the patterned GaAs surface shows a good surface morphology, which strongly indicates that this technology makes it possible to repeat crystal growth and surface patterning.