Thermal control is one of the most important issues in designing a spacecraft. A light, reliable, and inexpensive thermal-control device is strongly demanded. We have developed a new thermal-control device, Smart Radiation Device (SRD), based on a metal-insulator transition of (La, Sr) MnO₃ perovskite-structure oxides. The emittance property of the device changes significantly at the transition temperature near room temperature; high at high temperatures whereas low at low temperatures. Thus, the device automatically controls the emmisive heat transfer from the spacecraft without the need for any mechanical parts and electrical power. The practical validity of the device was demonstrated in the recent flight of a spacecraft “HAYABUSA”. The developed oxide-thin-film radiator also greatly reduces the weight and production cost of the thermal control devices.