Atomically ordered array of Cu atoms on a clean single-crystal Cu (100) surface has been observed on both occupied and empty sample states by scanning tunneling microscopy in ultrahigh vacuum at room temperature. An average corrugation amplitude of similar to0.4 Angstrom is found in the empty state while it is only similar to0.06 Angstrom in the occupied state. Atomic spacing is measured and the results are in good agreement with the known copper interatomic distance of 2.6 Angstrom.