Abstract: Different from FET, thermal properties measuring of MMIC by conventional electrical method is unsatisfactory because MMIC power amplifier integrates electronic components such as resistor, capacitor, etc. Related measuring methods and devices are studied and designed in this paper. A novel method based on the conventional electrical method is used to measure the thermal properties of two GaAs MMIC power amplifiers, and both the layer constitution of thermal resistances and the transient thermal resistances are obtained. Structural function comparison reveals the difference of Sn-Au die between the two test samples. The accuracy of the presented method is similar to IR measuring (10 μm) method, but the presented method has advantages such as convenience and quickness.