In this paper, the new plug-in hybrid system installed in the new Prius Prime is described. This system is based on TOYOTA Hybrid System II (THS II), and the EV driving performance is greatly enhanced from the original THS II. Although the EV driving performance is enhanced, environmental performances of the vehicle, the electricity consumption and fuel consumption, are the top level in midsize cars. In order to realize it, several units and systems are newly developed. The high capacity Li-ion battery is developed for the long EV distance and large EV power, and one-way clutch is installed in transaxle to realize “Dual Motor Drive” system. By using it, EV driving could be realized by not only motor driving force but also generator driving force. Additionally, the power control unit (PCU) is strongly enhanced to realize high EV output power from the Li-ion battery. And the EV driving force is appropriately modified to realize good EV driving feeling. The battery warming system is developed to realize keeping good battery temperature under cold condition. With heat pomp climate system, EV driving could be realized under cold condition. For charging, charging power of the new charger is 1.6 times as large as that of previous one in order to decrease the charging time. And the timer charging could be used with pre-climate system and the battery warming system, drivers could feel comfortable driving condition when they ride on the vehicle with this function. The configuration and specification of this new plug-in system is described in this paper.