We have studied the radiation of a double-walled carbon nanotube (DWNT) filament with a length of 4.5 mm and a diameter of 10μm by applying an electric current through the filament. The DWNT filament starts emitting incandescent light at voltage U = 6 V. Emission spectra of the DWNT below temperature 1250 K can well be fitted to those of the blackbody radiation. The intensity of the incandescent light shows an exponential dependence on the voltage applied on the DWNT filaments. The resistance of the DWNT filaments is very stable at high temperatures between 900 and 1250 K during the emission of light in the experiments.