Transmission losses of two types of dielectric-coated rectangular waveguide are analyzed theoretically to transmit linearly or circularly polarized C02 laser light. When ZnS and PbTe are chosen as coating materials, waveguide losses are reduced by ˜ 1 order of magnitude by increasing each pair of coating materials. Phosphor bronze rectangular waveguides with small aspect ratios and various cross sections, whose inner walls are coated with a single PbF2 layer, are fabricated by using vacuum-evaporation and assembly techniques. Transmission losses of straight and bent waveguides are measured for coherent C02 laser light as well as for incoherent infrared lights. Straight waveguide losses of 0.1 dB/m including a coupling loss from a laser, are obtained at a 10.6-xm wavelength for 1-m-long and small-core waveguides.