Chlorine‐enhanced GaAs maskless etching has been performed with a novel focused ion beam etching (FIBE) system. The system is composed of an air‐locked ultrahigh vacuum chamber, a 30‐keV Ga+FIB column, and a fine nozzle. The nozzle irradiates a high‐density Cl2flux on a desired, small area of the sample while retaining a sufficiently low surrounding gas pressure for stable Ga+FIB emission. Condensed Ga residues, appearing on the etched surface with no Cl2gas, could be suppressed under Cl2gas irradiation. Highly chemically enhanced sputtering yields (up to 50 GaAs molecules per incident ion) were obtained by selecting the optimum relationship between scanning time and Cl2gas pressure. At the maximum yield, a deep groove (about 6 &mgr;m) with a smooth surface was obtained by line‐scanning FIBE. The etching was applied to laser‐mirror formation of an AlGaAs laser. A vertical mirror facet, fabricated in advance by a reactive ion beam etching, was trimmed about one micron thick by line‐scanning FIBE. Light output versus current characteristics did not change before and after FIBE and the etching has been shown to be useful for laser‐mirror formation.