The gamma irradiation effect in indium phosphide (InP) heterojunction bipolar transistors (HBTs) is studied in this paper. The direct-current (DC) and alternating-current (AC) characteristics are investigated before and after an irradiation dose of 10 Mrad(Si). The main effects of gamma irradiation for InP HBTs are the following: increase of forward Gummel base current at low bias regime, decrease of common emitter collector current, increase of junction capacitances, and decrease of cutoff frequency. The Keysight model is adopted to describe behaviors of InP HBTs including DC and AC behaviors. The Keysight model parameter values are extracted before and after irradiation, in turn to study the physical mechanisms responsible for irradiation-induced degradation in InP HBTs.