This paper describes the thermal design and analysis of the Clouds and the Earth's Radiant Energy System (CERES) instrument with emphasis on the detector thermal design. CERES has been selected as a payload on the Earth Observing System (EOS-AM-1) spacecraft which is the principle component of NASA's Mission to Planet Earth. Several broadband scanning radiometers will measure emitted and reflected energy from the surface of the Earth and the atmosphere to provide a long-term consistent database of the Earth's radiation balance and ultimately the effect of clouds on the Earth's climate. CERES requires precise active thermal control of the thermistor bolometer detectors to measure extremely small temperature changes induced by incident radiation. A detailed thermal design and analysis of the CERES detectors was completed to ensure minimal thermal disturbances from environmental heating and instrument temperature variations. The overall instrument thermal design incorporates passive thermal control techniques to reject approximately 50 watts of heat from the instrument. The thermal design currently includes adequate temperature and power margins for all environmental conditions and instrument modes of operation.