A subtraction process of atomic layer manipulation was achieved in GaAs by alternately feeding an etchant of Cl and applying a low energy Ar ion beam to the GaAs substrate. The etching rate saturates exactly at one molecular layer per cycle and is independent of etchant feeding rate and the energetic ion beam flux. The etched profile is extremely smooth and the number of defects induced by the etching is very small in comparison with that of conventional etching techniques.