In situ atomic force microscopy (AFM) was used to observe the surface structure change during electrodeposition of Cu on p-GaAs(100) surface in HCl solution. How the electrodeposition of Cu proceeded was strongly dependent on the structure of the substrate. In the portion where the surface was relatively smooth, Cu tended to deposit first, forming randomly distributed Cu clusters followed by the three dimensional growth of the clusters. On the other hand, when the surface already had some structure,lu deposited along the structure of the substrate.