A gearbox for the transmission of an aircraft propulsion system has complex requirements, such as a gear rating of AGMA 12 or higher, application of aviation bearings, stability of the lubrication system at various aircraft attitudes and environments, compact and lightweight design, and maintenance of a high-grade quality system. These characteristics distinguish it from general gearboxes for automobiles, construction machinery, and agricultural machinery. Research and development on aircraft gearboxes became popular domestically to a limited extent after 2010 because of difficulties associated with the design and fabrication technologies as well as limited demand. In this article, we describe the development of a gearbox with a high reduction ratio that is connected to the output of a turboshaft engine of an unmanned aerial vehicle and transmits power with over 95% efficiency while decelerating to 1:10. The requirements for gearbox development are first derived for design/analysis/manufacturing, and a test rig is developed to evaluate its performance and endurance; finally, the test results are described.