The development of a power electronic circuitry for a flywheel energy storage system is discussed in the following aspects. First, due to the nature of permanent magnet brushless DC motor/generator, the operation of three-phase voltage source inverter/rectifier can be simplified to that of a bi-directional DC-DC converter, allowing the use of mostly analog control. Second, there is a problem associated with the existing six-step brushless DC motor/generator control in the generator mode. A twelve-step control scheme is proposed to solve this problem. Third, high-switching frequency is necessary for the flywheel charger/discharger in order to reduce the size/weight of the system and to synthesize the high-frequency motor/generator current waveforms. A working prototype demonstrates that a high efficiency can be achieved at 1 ~O-kHz switching frequency by the innovative ZVT soft-switched three-phase inverter/rectifier. / Master of Science