This paper presents an approach for designing hybrid scooter transmissions with planetary gear trains. The basic concept is to combine two planetary gear trains into a split-power system; so, by incorporating clutches and brakes when engaged, the system becomes a hybrid scooter transmission. The transmissions combine the power from two power sources, a gasoline engine and an electric motor. The systems use four different modes in order to maximize the performance and reduce emissions: electric-motor mode; engine-mode and engine/charging mode; power-mode; and regenerativebraking mode. Kinematic and power flow analyses are performed. According to the analysis results, formulae for the speed ratio of the transmission are derived and a procedure for identifying the arrangements with no internal recirculation of the split power system is developed. Sixteen possible configurations of hybrid scooter transmission satisfying the design requirements are presented. A design example is used to illustrate the design procedure and operation principle of the transmission. (c) 2006 Published by Elsevier Ltd.