In order to elucidate the formation process of a self-assembled monolayer (SAM) of alkylthiophene, a dodecylthiophene film on a gold substrate was analyzed by FT-IR spectroscopy. We found that dodecylthiophene successfully formed the SAM on the gold substrate with three stages in the formation process. In the primary stage, dodecylthiophene adsorbs uniformly on the substrate with the parallel orientation until full coverage is achieved. In the following stage, however, the alkyl-chain moiety aligns up to the normal direction against the substrate, and condenses to a close packed form; simultaneously, the thiophene moiety keeps the parallel orientation. In the final stage, the thiophene moiety orients perpendicular direction in accordance with the alkyl-chain. Our findings indicate that the intramolecular tilt and interaction play important roles in the orientation of the moiety and in the cooperative alkyl-chain orientation, respectively.