The wide application of terahertz technology demands accurate and traceable measurement of terahertz radiometry. The first international terahertz power comparison took place under the guideline of the consultative committee for photometry and radiometry of the international committee for weights and measures. A highly absorptive terahertz radiometer is fabricated for the comparison and the uncertainty of terahertz measurement are analyzed and experimentally investigated, for realizing the terahertz measurement accurately and guaranteeing the internationally equivalence and mutual recognition of Chinese terahertz measurements. Also, for confirming the measurement accuracy and uncertainty analysis of the terahertz radiometer, a terahertz radiometers with specular reflection is constructed, and the comparison measurements with the highly absorptive terahertz radiometer is performed. The self-made terahertz radiometer is used in the first international terahertz comparison, and excellent agreement is obtained.