The Korea Aerospace Research Institute is planning to launch a satellite for the Korea launch vehicle’s low orbit test in 2021. An electrically heated thermoelectric generator (ETG) with a mass of 850 g (small ETG) will be one of its selected payloads for this test flight. To assess and assure the design heritage and the reliability of the ETG in the space environment, a small ETG will be tested at the top of the satellite for more than a year. In this study, the design for a small ETG with 10 W(thermal) of heat input was developed through simulations such as heat transfer analysis, modal analysis, thermoelectric analysis, and structural analysis. Then a mock-up was fabricated and basic performance tests were carried out to be ready for the space environment test.