Abstract: Frequency is one of the key parameters of terahertz radiation, and accurate and traceable measurement of terahertz frequency is crucial for research and applications of terahertz technology. In order to achieve accurate measurement and traceability of terahertz laser wavelength, we have developed a terahertz interferometer to measure the wavelength and frequency of terahertz radiation sources. We constructed Fabry-Perot and Michelson interferometers to measure the wavelength and frequency of terahertz radiation sources, including backward wave oscillator terahertz sources and terahertz frequency multipliers. We propose using high-resistivity silicon plates as terahertz beam splitters, which not only have a simple structure, but also enable wide-band wavelength and frequency measurements. Using a single silicon plate as the beam splitter, we achieved ideal measurement results across a wide frequency range from 90 GHz to 800 GHz. By performing Fourier transforms on the measured terahertz interference patterns, we obtained terahertz frequency information. We calibrated the two terahertz interferometers at frequencies of 100 GHz and 300 GHz using terahertz frequency combs, enabling accurate measurement and correction of the radiation wavelength from the terahertz sources. This allowed traceability of the interferometer measurement results to the International System of Units (SI). We also performed uncertainty analysis on the measurements. Terahertz interferometry provides a simple and convenient approach to measuring terahertz wavelengths, and will find wide applications in terahertz metrology.