The conformation of the carboxylic group of fatty acids in a Langmuir (L) monolayer film on water is described in relation to the aggregation property of the hydrocarbon chain. Polarization-modulation infrared reflection absorption spectra (PM-IRRAS) of L films of heptadecanoic acid (C-17), octadecanoic acid (C-18), and nonadecanoic acid (C-19) exhibit systematic spectral changes in both the C-H and C=O stretching vibration regions. Through a stabilization analysis of the L films at a high surface pressure, the C-19 L film has been found outstandingly stable exhibiting no film shrink, while the other two compounds exhibit a large shrink at high surface pressure. By taking into account the uniquely high aggregation property of the hydrocarbon chains of C-19, the three major bands arising from the C=O stretching vibration mode propose three types of molecular conformations about the carboxylic group, which are elucidated by a balance of the hydration of the carboxylic group, the chain length of the hydrocarbon chain, and the surface pressure.