Microwave primary noise sources are always constructed with Planck's law. In microwave frequency region a matched load keeping in a stable temperature radiates noise power whose density is proportional to its physical temperature. The evaluation method of the primary noise source accounted for the distributed loss and temperature was presented by Stelzried in 1965. In this paper an extension evaluation formula including the influence of the reflection is deduced and the error caused by the reflection is discussed. The results show the influence is within 1 ie if the reflectivity is in the range of 0 to 0.03 as for the WR28 noise source.