Abstract: Contents of nitrogen, carbon and hydrogen in carbon fiber were determined by the Vario EL Ⅲ elemental analyzer. The optimized experimental conditions were as follows: ① amount of sample: 2.1-2.4 mg; ② time of oxygen dosing: 120 s; ③ temperature of combustion tube: 980 ℃. Acetanilide was used to calculate the daily factor. The developed method could be applied to samples of carbon fiber and graphite, giving the contents of carbon in the range of 88.05%-99.46% with less than 0.30% of nitrogen and hydrogen in most of the samples.