An electrochemical atomic force microscope was employed to observe the surface structure of p-GaAs(100) in H2SO4 solution and of electrodeposited Cu on p-GaAs(100) in CuSO4 + H2SO4 solution with atomic resolution. The atomic arrangement of GaAs(100)-(1 x 1) structure was observed in H2SO4 solution after the electrode was kept in the negative potential region for 5 min to remove the surface oxide layer, The atomic image was clearer in the potential region where a small cathodic current flowed. An atomically ordered structure of closed packed hexagonal arrangement corresponding to the bulk Cu(lll) face was observed at the surface of the Cu deposits on p-GaAs(100). The direction of the atom rows of the Cu deposits varied from grain to grain, suggesting that the Cu deposition on the GaAs surface was not influenced by the orientation of the underlying GaAs(100) surface. The deposition and dissolution of Cu on p-GaAs(100) in CuSO4 + H2SO4 solution were followed by AFM in real time at the atomic scale during potential cycling. The atomic image was resolved at the negative potential region where Cu deposited on the GaAs surface, but was completely lost at potentials more positive than +0.05 V as the dissolution of the bulk Cu deposits took place. The atomic image was recovered when the potential was returned to negative (-0.03 V) as Cu deposited on the GaAs surface again.