With the aim of understanding the nature of the S -Au(111) bonding in organosulfur/Au(111) self-assembled monolayer (SAM) systems, orbital interactions in the adsorption of methanethiolate (-SCH3) in various binding sites of a three-layer slab model and an Au-42 Cluster model of Au(111) surface are investigated. The methods of choice are crystal orbital overlap population (COOP) and crystal orbital Hamilton population (COHP) analyses for a periodic slab model and fragment molecular orbital (FMO) analyses for the cluster model. The origin of the S -Au(111) bond and the binding site preference are discussed from the viewpoint of orbital interaction. The site preference is in the order of three-fold hollow (fcc and hcp) > bridge > on-top. The second layer Au atoms have little influence on the S-Au(111) bonding, and adsorptions, to the fcc and hcp, sites are almost identical with respect to energy and S-Au bonding nature. Although sigma-type S-Au orbital interactions dominate the S-Au(111) bonding in the on-top model,pi-type S-Au orbital interactions play an important role in the bridge, fcc, and hcp models. FMO results,explain the vertical S-C bonds in the hollow models and the tilted S-C bonds in the on-top and bridge models.