Abstract: Thermal simulation is one of the design problems for millimeter-wave medium-to-high power amplifiers. Good thermal configurations were performed for the system to ensure that the active devices would be operating below their rated maximum operating temperature. Numerical simulation based on finite element method is an important tool in the thermal analysis of millimeter-wave power amplifiers. A three dimensional thermal model of a 6 W solid-state integrated power amplifier which is combined by using 1×4 monolithic millimeter-wave integrated circuit (MMIC) chips at Ka-band was built with ANSYS to calculate the temperature distribution under air free convection condition. Good agreement between simulation and experiment and which demonstrated the effectiveness of the thermal model. The simulated results provide a theoretical basis for thermal design of millimeter-wave power amplifiers.