The conformational change of bifunctional C30DME, alpha,omega-13,16-dimethyloctacosanedioate dimethylester, with changes of surface pressure at the air-water interface was investigated. The surface pressure (pi)-area (A) isotherm indicates that the conformation of C30DME at the air/water interface changes from linear to reverse U-shape as surface area is compressed. The two hydrophilic ends anchored on the water surface and the hydrophobic chains rise up in the air and generate a reverse U-shape as the surface pressure increases. To confirm the formation of a reverse U-shape, we examined the thickness of the Langinuir-Blodgett film with a surface plasmon resonance spectrometer and an ellipsometer. Theoretical calculations coupled with experimental observations showed that the conformational change depends on the rotational sites in the middle of the structure, the 13 and 16 positions of the hydrocarbon, and the energy of the conformational transition is about 1 kcal/mol.