In this paper, the uncertainty in the calibration of coaxial thermal noise sources using a noise figure measuring (NFM) equipment is evaluated. Contributions to the uncertainty such as the calibration uncertainty of the standard noise source, mismatch, measurement of adapter efficiency, ambient temperature variation, and repeatability are evaluated in the frequency range of 10 MHz to 18 ㎓. Results show that the expanded uncertainty(k=2) is 0.23 ㏈ for the noise sources of 5 ㏈ and 15 ㏈ ENR, and 0.27 ㏈ for those of 21 ㏈.