The structure of alkyl monolayers on the H-terminated Si(111) surface under the external electric field was investigated by molecular simulation method. Molecular mechanic calculations showed that the energy per alkyl chain reduces linearly with increasing of carbon number in alkyl chains from two-carbon to eighteen-carbon. When the external electric field was added between the optimized alkyl chains, the calculated energy per chain is coincident with the experimental current density after alkyl-modified silicon crystal was considered as the semiconductor. The conclusion is that molecular simulation can provide other information and be considered as an adjunct for the experiments from another investigation way. (C) 2004 Elsevier B.V. All rights reserved.