Abstract: Accuracy of blackbodies' reference source radiance is one of the crucial factors that determine the accuracy of infrared sensor's absolute radiometric calibration. To improve the measurement accuracy of blackbodies' radiance, a infrared standard radiometer was developed to calibrate blackbodies' radiance directly. The principle and the opto-mechanical design of the instrument were described in detail, and the radiometer was calibrated against a high accuracy water-bath blackbody. The experimental results demonstrate that radiometer's instability is less than 0.03% with in one hour and the uncertainty is less than 0.22%. This corresponds to a brightness temperature uncertainty of 73 mK at 308 K. The radiometer possesses the advantages of system-level measurement and high-precision tracing standard, and by which the blackbodies' radiance can be traced to the radiant standard in lab.