The Smart Radiation Device (SRD) is a thin and light tile whose infrared emissivity is varied proportionally by the temperature of the radiator. Bonded only to the external surface of the spacecraft's instruments, it controls the heat radiated to deep space without electrical or mechanical instruments used for changing emissivity. Its function is similar to the thermal louver which has been used for a lot of spacecraft, but the SRD is lighter than it. Thus, by using this new device, we can control the temperature of the instruments on the spacecraft more easily. The materials of the SRD are La0.825Sr 0.175MnO3 and La0.7Ca0.3MnO 3. In this paper, design and preliminary test results of the SRD will be presented. The optical properties for the materials of the SRD, such as the total hemispherical emittance and the solar absorptance, have been measured. In addition the degradation by protons has been investigated. © Copyright 1999 Society of Automotive Engineers, Inc.