In order to investigate the formation process of self-assembled monolayers (SAM) of alkanethiol, we analyzed the molecular organization by Fourier-transform infrared reflection-absorption spectroscopy (FT-IR-RAS). The spectra suggested that molecular structure of the SAM on gold (111) surface is dependent on the immersion time. Alkanethiol in the SAM aligns normal to the substrat surface. A stable SAM embedded a close packed monolayer by the van der Waals interaction among alkanethiol and surface force from the gold substrate.