In 1998, the first element of the Earth Observing System, the EOS-AM1 spacecraft, will be launched into a sun-synchronous polar orbit. The EOS-AM1 platform will include five instruments that will form the payload used to provide information on global climate change. The spacecraft will also include a dual frequency (Ku- and S-band) High Gain Antenna (HGA) which will serve as the primary system for transmitting the science data and spacecraft telemetry & command data to the ground via the TRDSS (Tracking and Data Relay Satellite System) network.The HGA assembly consists of a Casgrain reflector subassembly rigidly attached to a box enclosure which contains the Ku-band transponder. The HGA transponder has ten electronic units providing two completely redundant power feeds.Since the TDRSS satellites and the EOS spacecraft are orbiting in different flight paths, data transmission is maintained by continuously repositioning the HGA assembly via a dual-axis gimbal mechanism. The multiple pointing configurations possible and the particular tracking characteristics of the HGA assembly led to a challenging thermal design and a complex analysis scheme.The HGA assembly system thermal analysis includes transient thermal analysis for all operational and non-operational modes, with the antenna in the deployed and stowed configurations.The results of the analysis confirm that the overall thermal design adequately protects the HGA antenna from its environment. The flight model of the HGA assembly is currently being integrated and is scheduled to go into thermal balance testing in late 1995.