This paper presents a simple procedure to extract an electrothermal model for MMIC power amplifiers. The model accounts for the DC and RF performance of MMIC complex structures for-med by multiple transistors combined in different amplifying stages. All the steps taken to perform the thermal and electrical experimental characterization to obtain the model are explained. The extracted model was validated by comparing measured and simulated data of a commercial GaAs FETMMIC power amplifier. Is was also used to predict the hot spot temperature and position over a wide range of working conditions.