The method to evaluate the dihedral angles of a polypeptide using IR absorption spectroscopy was applied to a polyproline type II (PPII) structure. We have prepared 1PPPPPPG7-NH2 with the two successive residues labeled by 13C[dbnd]18O. The doublet of the 13C[dbnd]18O stretching band due to the two successive labeled residues was observed, which comes from the vibrational coupling between the labeled residues. The differences in the peak wavenumber and the intensity of the doublet band were analyzed using the GF matrix method and ab initio molecular orbital calculations. The obtained dihedral angles of the labeled residues were compared with the reported values.