Abstract: We presented a technique for frequency calibration of terahertz time-domain spectrometers (THz-TDS) using absorption lines of carbon monoxide (CO). By comparing the peak positions of absorption lines between measured results and standard data, the linearity error caused by the optical delay line was modified so as to achieve the frequency calibration. After modifying, the mean error of the peak positions measured by our THz-TDS was 3.1 GHz. The results show that CO is suitable for frequency calibration of THz-TDS.