Infrared (IR) spectra of the Langmuir monolayers of stearic acid were simulated by the molecular dynamics (MD) method, for the first time, where the translational, rotational, and vibrational degrees of freedom of all atoms in the monolayer were considered. The IR spectra simulated by this method completely reproduced surface area dependence of the monolayer structure and the chain axis orientation. The characteristics of the spectra were also consistent with the chain orientation evaluated by in situ experimental techniques. In addition, the simulated spectra gave evidence of no area dependence of the head group orientation in the monolayer.