The heterojunction between Si(111) semiconductor and the well-ordered methyl monolayer was formed to investigate the channel conduction at the surface in the heterojunction structure. With the presence of water, the methyl monolayer/Si heterojunction structure showed switching action with a high trance-conductance of 2.8 x 10(-3) A/V under the application of bias voltage, while without the presence of water, such a switching behavior was not observed. The methyl monolayer/Si heterojunction structure worked as a p-type mode with the presence of water at the modified surface.