We report a Monte Carlo simulation study of the self-assembly of 1,4-benzenedithiolate (BDT), tetrahydrofuran (THF), and their mixtures on a Au (111) surface. We use the grand canonical Monte Carlo method to obtain the equilibrium adsorption coverage. Canonical ensemble (NVT) simulation is then used to explore further the structural information of the equilibrated systems. Our results indicate that BDT molecules adsorb onto the Au (111) surface with one of the sulfur atoms bonded to Au atoms. THF molecules form clusters on the surface. For BDT-THF mixtures, BDT can selectively adsorb on Au (111) to form a monolayer, whereas the solvent THF molecules either float above BDT monolayer or occupy vacancies on the surface that are not covered by BDT molecules. BDT molecules adsorb on a Au (111) surface with an average tilt angle of about 18-35 degrees to the surface normal. The tilting angle decreases as the coverage increases. In addition, the BDT monolayer constitutes an ordered herringbone structure on the Au (111) surface, and the ordering pattern is insensitive to the BDT coverage. In comparison, the THF molecules exhibit amorphous structure on the Au surface. Interestingly, simulations indicate that the bonding behavior of BDT molecules on Au (111) is coverage-dependent. BDT bonds preferably on the Au top site when the surface coverage is low. As coverage increases, most BDT molecules bond on the bridge and fcc hollow sites.