FORMOSAT-3 program consisting of six small-satellites are used for weather prediction, atmospheric studies, and space weather monitoring. The satellites were injected into the parking orbit of 516 km and being transferred to final orbit of 800 km individually. Because electrical power is quite critical for micro-satellite, passive thermal control hardware is used as much as possible to save heater power and heaters are only designed and used to warm up critical components. All worst hot and cold cases are analyzed with corresponding beta angles, orbit altitudes, thermal environments, and operating modes. After conscientious trade-off studies among all worst hot and cold cases, radiator area and heater power are determined and finalized to meet all design requirements.