This study evaluates the thermal deformation of an ultralight antenna panel structure made of CFRP thin plate to examine its applicability to the 30m class Geostationary Precipitation Radar (GPR) satellite. This GPR satellite is a midway target to realize the future space solar power systems. The proposed antenna structure is made of three layers of materials; copper foil, dielectric, and CFRP. To design this structure ensuring the flatness required for the electromagnetic performance, the finite element model was established. The validity of the model was tested against the thermal test of the coupon specimen with a single antenna patch or strip. The thermal deformation of the antenna unit which consist of hundreds of antenna patches was then numerically examined. Furthermore, sensitivity studies regarding the layer thickness and the loading condition were conducted to improve the flatness of the proposed antenna structure.