This work investigates the effects of gamma irradiation on the residual phase noise performance of Sice HBTs. The excellent phase noise capability of SiGe HBTs is retained after 1 Mrad(Si) irradiation. The observed radiation hardness is attributed to the minor changes in the low frequency noise and device nonlinearities after irradiations, as well as the nonlinearity cancellation mechanism. The radiation-induced defects do not significantly add to the low frequency noise in these SiGe HBTs, even though they produce a large space-charge-region recombination component in the base current. The inherent excellent linearity of these SiGe HBTs makes the up-conversion from device low frequency noise to phase noise inefficient. and helps to retain the low phase noise after irradiations.