Anodic dissolution of a GaAs(100) face was investigated by in situ electrochemical atomic force microscope (AFM). While no surface structure change was observed at -0.6 V (vs Ag/AgCl) where no current flowed, dome structure on surface was removed, and flat surface was obtained after keeping the potential at 0 V (vs Ag/AgCl) where anodic current of approximately 150 muA cm-2 flowed. An atomically resolved AFM image was obtained in the flat region and shows the surface is dominated by a (111) face after the anodic dissolution.