Molecular mechanics and molecular dynamics simulations were performed to search for the optimal packing structure of C-18 alkyl monolayers on a Si(111) surface. The optimal molecular packing was found based on the consideration of three major factors: molecular substitution, substitution pattern, and molecular orientation on the Si(111) surface. Ab initio quantum chemical calculations were also performed to validate the force field used for the alkyl/Si(111) system. It was shown that the optimal packing structure was size-independent when it was extended to large systems. Molecular dynamics simulation results based on the optimal packing structure in a temperature range from 50 to 500 K agree with those from experimental measurements for various properties, including system and molecular tilt angles, film thickness, and gauche defects. The optimal packing structure found in this work is a basis for further molecular simulation study of alkyl monolayers on silicon for various engineering applications, such as adhesion and friction reduction in micro- or nanoelectromechanical systems.