In this paper, we describe the thermal characterization of a multifunctional hybrid photovoltaic panel for the conversion of sunlight into microwave that is used in a solar power satellite. We designed a practical configuration of the hybrid panel and investigated the thermal characteristics by experiment and numerical analysis. First, thermal-vacuum test using a prototype panel was conducted to obtain practical thermal resistances between each layers of the panel. Using the obtained thermal resistances, we then simulated daily temperature variation and thermal deformation during an orbit. The simulation results show that the deformation is maintained within allowable range in the present panel configuration; panel temperature is, however, too low when top and bottom surface do not receive sunlight.