We have investigated the oscillatory tunneling current-voltage characteristics on metal nanoclusters formed on Sb-terminated Si(100) surfaces by using scanning tunneling microscopy/spectroscopy. Through the systematic investigation on a variety of cluster configuration environments, we suggest that the lateral tunneling between adjacent clusters dominantly contributes to the occurrence of the single electron tunneling phenomena. In the single clusters formed on Si surfaces, we detected only current oscillations, which must be distinguished from Coulomb staircases. Those results strongly suggest that Coulomb staircases should not originate from the direct conduction of electrons through Schottky junction between the single clusters and Si substrates in contrast to other previous reports. (C) 2000 American Vacuum Society. [S0734-211X(00)02905-X].