We report a higher precision measurement of the refractive index of carbon dioxide using a frequency comb as the light source in a Mach–Zehnder interferometer setup. The experimental sensitivity can reach the level of $8.8\times {10}^{-9}$ . Taking into account the measurement accuracy of temperature and pressure, the experimental accuracy has a value of $1.2\times {10}^{-8}$ . The measurement result has a deviation from the commonly quoted result [Old et al., J. Opt. Soc. Am. 61, 89 (1971) ] by $6.4\times {10}^{-7}$ at $800\mathrm{nm}$ .