The structure of eight-carbon monolayers on the H-terminated Si(111) surface was investigated by a molecular simulation method. Molecular mechanics calculations showed that the best substitution percentages on the Si(111) surface were 50% for octene or octyne-derived monolayers and 40% for the styrene or phenylacetylene-derived monolayers. These values are in good agreement with the experimental results. After a two-dimensional cell containing alkyl chains and four layers of Si atoms was constructed, the densely packed and well-ordered monolayer on the Si(111) surface can be shown at the molecular level. At the same time, the thickness of the monolayers and the tilt angle of the alkyl chain were also calculated. Additionally, the theoretical calculations showed that the C=C bond of the alkyne only reacts once with the H-terminated Si(111) surface, that is only one Si-C bond per organic molecule is formed on the Si(111) surface, which verifies the experimental results. It is thus shown that molecular simulation can provide otherwise inaccessible microscopic information at the molecular level, and may be considered as a useful adjunct to experiments.