Abstract: The sample (0.100 0 g) was placed into a crucible, and 0.50 g of iron scurf flux and 1.50 g of tungsten tin flux were added into the crucible. The mixture in the crucible was burning at 400℃ for 1h in a muffle furnace. After cooling, HNO₃ (1+9) solution was added slowly in portions into the crucible until free of bubbles. After drying at 110℃, high frequency combustion-infrared absorption spectrometry was applied to determination of fixed carbon in the sample. 2 CRMs (GBW 03118, GBW 03119) were analyzed by the proposed method, giving results in consistency with the certified values, and RSDs (n=6) of 1.1% for GBW 03118 and 0.30% for GBW 03119.