A carbon nanotube cryogenic radiometer (CNCR) has been fabricated for electrical-substitution optical power measurements. The CNCR employs vertically-aligned multiwall carbon nanotube arrays (VANTAs) as the absorber, heater, and thermistor, with a micro-machined silicon substrate as the weak thermal link. Compared to conventional cryogenic radiometers, the use of CNCR is simpler,more easily reproduce and disseminated, orders of magnitude faster, and can operate ofver a wide range of wavelengths without the need for a receiver cavity. We describe initial characterization results of the radiometer at 3.9 K, comparing electrical measurements and fiber-coupled optical measurements from 50 υW to 1.5 mW at the wavelength of 1550 nm. We find the response to input electrical and optical power is equivalent to within our measurement uncertainty, which is currently limited by the experimental setup (large temperature fluctuations of the cold stage) rather than the device itself. With improvements in the temperature stability, the performance of the CNCR should be limited only by our ability to measure the reflectance of the optical absorber VANTA.