A model for computing the thermal transient response of a diamond-heat-sinked IMPATT diode has been formulated as a means for accurately predicting the degree of heating or cooling of the junction when the diode is pulsed into or out of avalanche. The model consists of an electrical network analog for the heat conduction process, and the transient analysis of this network has been performed using the IBM Advanced Statistical Analysis Program (ASTAP). Also incorporated into the model are the results of previous numerical determinations of steady-state temperature distributions in IMPATT diamond heat sinks. The thermal responses for diode turnon and turnoff and for power surges have been found for several different designs of IMPATT diodes, both Si and GaAs. Turnon transients calculated with this model have been compared with transients calculated by a published method [11] involving a transcendental equation. The two models were roughly in agreement. However, because the previously published method neglects the heat flow path through the chip, it yielded lower values than the network analog model described here for the junction temperature in the first few microseconds after turnon of the diode. The results of these calculations showed that the transient response varied depending on the size of the chip and that significant temperature changes occurred in time intervals ranging from less than 0.1 µs to several microseconds for practical diodes. The results also showed that a description of the transient response in terms of a simple time constant is not meaningful, because the early response does not approximate an exponential curve. To provide a means for making quite accurate desk calculations of diode thermal transients, two approximations have been derived which can be used without computer programs.