A novel ultrahigh vacuum (UHV) sample transfer system was constructed for the purpose of transferring Cl2‐gas etched GaAs samples from aninsituprocessing UHV multichamber to a scanning tunneling microscopy (STM) chamber. First, the performance of the sample transfer system was confirmed by an UHV‐STM observation of an As‐stabilized (2×4) structure of a molecular‐beam epitaxially (MBE) grown GaAs (001) surface. Next, four kinds of Cl2‐gas etched samples were prepared in the UHV multichamber, and an UHV‐STM observation of these etched surfaces was carried out. A strong dependence of the surface roughness on the etching temperature was observed; a 200 °C Cl2‐gas etched GaAs (001) surface exhibited a corrugation amplitude of several nanometers, which was about 1.5‐times smaller than that of a 70 °C etched surface. Through annealing under an arsenic pressure at 600 °C which is a typical temperature for MBE growth, the smoothness of the etched surfaces remarkably improved; the corrugation amplitude of the 70 °C etched surface decreased by a factor of 2, and that of the 200 °C etched surface decreased by a factor of 3–4. As a result, the 200 °C etched surface has become very smooth, being only about 1.5‐times rougher than the MBE grown surface. This indicates the usefulness of theinsitufabrication processes, since an interface formed by Cl2‐gas etching and following MBE regrowth has been guaranteed to be nearly as smooth as an as‐grown surface.