The adsorption of Sn and Zn on a Si(111)-7 x 7 surface was studied by a scanning tunneling microscopy (STM). Single Sn atom is adsorbed in the faulted and unfaulted half unit cells at low coverage. When the half unit cells comprising an adsorbed single Sn atom are separated each other, no diffusion of the Sn atom to the neighboring half unit cell takes place at room temperature although the Sn atom undergoes rapid migration over the three center Si-adatoms in the half unit cell. When an adjacent half unit cell adsorbs a Sn atom, however, the two single Sn atoms in the two adjacent half unit cells are rapidly agglomerated to a Sn-2-dimmer, that is, the diffusion barrier is lowered at the border. In addition, the single Sn atoms in the adjacent two half unit cells undergo replacement with a center Si-adatom, and a Si-4-cluster is formed. On the other hand, adsorbed Zn atoms on a Si(111)-7 x 7 surface prefer to form one Zn-3-dot in each half unit cell. As a result, a honeycomb layer composed of Zn-3-cluster is established as increasing the coverage. These results suggest that the adsorbate-adsorbate interaction plays important role in the formation of the precursor state, which is responsible for the feature of the surface reaction and catalysis. (C) 2003 Elsevier Science B.V. All rights reserved.