Dielectric properties of I-III-VI2-type ternary chalcopyrite semiconductors, including linear and second order nonlinear optical susceptibilities at 10.6 mm, have been quantitatively studied from the chemical bond viewpoint. Contributions from each type of constituent chemical bond, i.e., I u VI and III u VI bonds, to the total linear and nonlinear optical properties of these compounds at 10.6 mm have been theoretically determined. The chemical bond method quantitatively expresses the trends in the dielectric properties of these compounds, which is helpful for carrying out modeling of their properties.