In the context of global warming and searching for alternative sustainable mobility, hybrid and electric vehicles have been seeing as a promising solution. The majority of these vehicles are power-split hybrid vehicles using planetary gear as speed coupling. This paper studies the kinematic behavior of this type of coupling by extensive literature research about hybrid and electric vehicles, transmission gearbox, power-split control systems, and planetary gear. A method to calculate the mechanism kinematics is presented in order to determine torque, angular speed, power factors, global gear ratios, and efficiency distribution of a planetary mechanism. Also, analysis and comparison of the transmission used in the Chevrolet Volt and in the Toyota Prius are carried out. As a result, the paper shows the Volt gives preference to a series operating mode at low speeds, using in most situations only the Electric Motors to provide torque, while Prius, even in urban velocities, has to use an Internal Combustion Engines to avoid drops in EMs efficiency. Thereby, it provides a guide for engineers in several fields to easily understand the function of mechanical design and open possibilities to discuss new operation modes and control settings aiming to increase efficiency and performance in the hybrid and electric vehicle transmission.