In order to verify the performance of upper stage propulsion system designed to operate in the upper atmosphere, test facility which can simulate high altitude is needed. Cylindrical supersonic exhaust diffuser, which utilizes the momentum of exhaust gas, provides a simple means for providing a low pressure around the propulsion system. This paper describes sub-systems and specification of high altitude test facility developed for the test of KSLV-I kick motor. Performance of the facility has been successfully verified through five times of hot firing tests.