Modal filtering in nulling interferometer is based on the capability of single-mode waveguides to transmit only one complex amplitude function to eliminate virtually any perturbation of the interfering wavefronts. In this letter, we focus on realizing single-mode waveguide of chalcogenide glasses in the thermal infrared range 6–20 \mu m by burying a GASIR (Ge-As-Se) fiber core in the substrates of As-Se-S glasses system. To match the single-mode operation, GASIR fiber core of about 15-μm diameter is drawn, the fiber core is buried into As-Se-S substrates by heating two kinds of glasses to a suitable temperature. The propagation mode is tested by guiding a CO2 laser emitting at 9.3 μm into the waveguide. Single-mode out coming signal is obtained by an infrared camera.