Ion beam assisted etching of GaAs has been performed by continuous and pulsed Ga focused ion beam irradiation in Cl2 atmosphere, and etch yield and surface composition have been measured as a function of beam energy and Cl2 fluX. It was observed that pulsed irradiation results in a 10 times or more higher etching rate than continuous irradiation. From Auger electron spectroscopy, it was observed that the surface becomes As rich after the etching with the composition depending on the etching condition. The thickness of the surface altered layer extends 7-20 nm depending on the beam energy. These results are discussed based on rate equations.