Opening agricultural market progresses radically, reducing cost of high quality agricultural products becomes urgent. Aerial application using an agricultural helicopter helps precise and timely spraying and reduces labor intensity and pollution. The development of an agricultural helicopter was necessary for taking advantages of both technique and economy. In this study, as the first stage of developing an unmanned helicopter capable of 20kg payload, an engine was selected and a prototype transmission was designed for an agricultural helicopter. Prony type dynamo-meter was constructed, the engine was tested and then performance curve was obtained. The centrifugal clutch was engaged at the rotation speed of 3,500-4,000 rpm. Maximum power was expected at the engine speed of 5,900-6,200 rpm when adjusted at the optimal output. Based on the test results, the transmission was designed for driving main rotor shaft.