By investigating orbital overlap population (OOP) resulting from the interaction between adatom and substrate Si atom, and total density of states of the cluster, it is found that Si(lll) 31/2×31/2- Sn has a half filled surface band which makesthe surface metallic. But for Si(lll) 31/2×31/2-Al, its bonding surface states are occupied fully with electrons and the antibonding surface states are empty, which makes the surface appear to be semiconducting. These calculation results are consistent with the experimental ones. A clear description on the interactions between Al, Sn and surface Si atom is also presented in this paper.