The adsorption of one monolayer S atoms on ideal Si(100) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of adsorption systems of S atoms on different sites are calculated.It is found that the adsorbed S atoms are more favorable on B1 site (bridge site) with a distance 0.131 nm above the Si surface. The S, Si mixed layer might exist at S/Si(100) interface. The layer projected density of states are calculated and compared with that of the clean surface. The charge transfers are also investigated.