Many power switching devices are used in a hybrid vehicle (HV) and an electric vehicle (EV). The power electronics in the HV/EV system is reviewed. For future development of the HV/EV system, higher performances of the power devices than Si limit, for example, low on-resistance, high operation temperature, are strongly required. GaN power devices are promising candidate for the requirements. Developing GaN power devices are lateral structure and vertical structure. Recent progress of the GaN power devices are also reviewed.