We demonstrated that sum frequency generation (SFG) vibrational spectroscopy can distinguish different secondary structures of proteins or peptides adsorbed at solid/liquid interfaces. The SFG spectrum for tachyplesin I at the polystyrene (PS)/solution interface has a fingerprint peak corresponding to the B-1/B-3 mode of the antiparallel beta-sheet. This peak disappeared upon the addition of dithiothreitol, which can disrupt the beta-sheet structure. The SFG spectrum indicative of the MS1594 alpha-helical structure was observed at the PS/MS1594 solution interface. This research validates SFG as a powerful technique for revealing detailed secondary structures of interfacial proteins and peptides.