Introduction: More and more “Green Power” concepts have been applied to the development of sport equipments, ex. the bike trainer combined with power generator. Therefore, the purpose of this study was to determine the efficiency of electrical / bike power transmission in different cadences on bike trainer with generator. Methods: Six young male were recruited as subjects and performed on the bike trainer with generator with 60, 75, 90 105 rpm cadences and 50/21T, 50/16T, 50/12T gear transmissions by a random sequence. The power meter and the bike trainer were attached on the rear wheel, and the electrical power was calculated by the voltage and resistance of load connected to the generator. Results: There were significantly high correlations among speed, bike and electrical powers. In all gear transmissions, the efficiency of power transmission on 105 rpm was significantly higher than 60 rpm. In 50/12T gear transmissions, the efficiency of power transmission on 105 rpm was significantly higher than 75 rpm, and 90 rpm was also significantly higher than 60 rpm. Conclusions: The electrical power has increased by speed. The change of power transmission efficiency was obvious from low to high cadences in the higher gear transmission, but the effect of cadence would become higher in lower gear transmission.