We investigated the structure of adsorbed L-phenylalanine (Phe) on the glassy carbon electrode (GCE) as a function of electrode potential using infrared-visible sum frequency generation (SFG) vibrational spectroscopy. Experiments using partially deuterated Phe confirmed that SFG signal originates from the CH stretch modes of the methylene group (CH2) in Phe. Quantitative analysis of SFG spectra revealed that Phe molecules adopt a conformation in which the dipole contribution of the asymmetric mode of the methylene group becomes greater than the symmetric mode with respect to the surface normal at highly positive electrode potential.