The early external active thermal control system (EEATCS) is used to perform the cooling of the United States Laboratory (USL) during early assembly stages of the International Space Station (ISS) to support assured early research. The EEATCS provides the ability to transport the thermal load generated by the USL to space by thermal radiation via two photovoltaic radiators (PVR). The EEATCS can be described as an indirect heat rejection system composed of two ammonia flow loops. Each loop contains accumulators, radiators, a heat exchanger, piping, and an independent pump flow control subassembly (PFCS). The PFCS contains the flow control valve (FCV), which controls the radiator bypass flow in each loop.Test performance of the EEATCS was accomplished at Kennedy Space Center (KSC) to identify its capabilities and characteristics under several designed test conditions. These test conditions challenge the EEATCS to respond to adverse environment and operational conditions. The test results showed that the EEATCS had excellent controllability and stability through all its test conditions.The test data was reduced and compared with an EEATCS SINDA/FLUINT model that showed very good agreement with the test data. This verification of test and numerical data gave a high degree of confidence and understanding in the hardware and software used in the EEATCS for its support to the ISS.