The infrared (IR) absorption bands due to peptide bonds (amide bands) have long been used to determine the secondary structure of a peptide and to analyze intra- and intermolecular interactions between amides. In the present study, the dihedral angles of a residue in peptides have also been evaluated using the amide I IR band of two successive residues with isotope labeling. The two successive residues labeled with the13C and18O isotopes give the doublet amide I IR band and the intensity ratio (Rint) and the difference in peak position (¦ν) of the doublet band were analyzed using GF matrix and ab initio molecular orbital calculations. We obtained the two-dimensional calculation maps of Rint and ¦ν against the two dihedral angles. The crossing point of the curves of Rint and ¦ν is the two dihedral angles of the measured residue. The evaluated dihedral angles of the simple peptides are compared with the reported values. We discuss the limitation and the application of the present method to biopolymers from the obtained results.