We studied the adsorption state of dimethyl disulfide (DMDS), methylthiolate (MeS), ethylthiolate (EtS), and butylthiolate (BuS) molecules on Au(111) surfaces by means of the density functional theory (DFT) within a generalized gradient approximation (GGA). It turns out that the MeS adsorption is more stable than the DMDS adsorption and that the most stable adsorption site for MeS is the bridge site slightly off-centered towards the fcc-hollow site with its S-C bond tilted from the surface normal by 53^o. The fcc-hollow site adsorption is less stable than that of the bridge configuration. The bridge configuration excellently reproduces experimental HREELS as well as core level shift results. The energy difference between the bridge and the fcc-hollow configurations depends on the coverage, indicating that there is a rather strong adsorbate-adsorbate interaction, while its dependence on the alkyl chain length is small. Vacancy formation upon alkanethiolate adsorption on Au(111) is suggested from our GGA calculations.