The growth process of a self-assembled monolayer (SAM) of thiophene on Au(111) was studied by Fourier-transform infrared reflection absorption spectroscopy (FT-IR-RAS). FT-IR-RAS measurements of the SAM revealed two stages of growth process and the polymorphism. In the primary stage, thiophene in the SAM orients parallel to the gold substrate. In the second stage, thiophene reorients to the perpendicular position against the gold substrate. The phase transition of the molecular orientation in the growth process is mainly controlled by the balance between the intermolecular and the thiophene-gold interactions.