This paper describes the motorized turbo compressor, which is a key technology for reducing the size of the fuel cell system for the Clarity Fuel Cell.The oxygen needed for fuel cell power generation is sent into the fuel cell by compressing the air from the atmosphere by a compressor. The conventionally used Lysholm compressor needed numerous sound absorbers, such as silencers and covers, to help achieve quietness when driving. Therefore, changing to a turbo compressor enhanced quietness and helped to eliminate or reduce the size of these auxiliary sound absorbers. Furthermore, a two-stage supercharging structure was used and the air pressure supplied to the fuel cell was increased to 1.7 times the previous air pressure. This increased the fuel cell power, which enabled to reduce the number of cells needed, and reduced the needed humidification amount which enabled to reduce the size of the humidifier. These enhancements helped to reduce the system size.In addition, air bearings that are clean (no oil contamination) and can support higher speeds were used as the bearings for the turbo compressor.A dedicated compact inverter was also developed, which facilitated installation in the vehicle, and the use of sensor-less control helped to eliminate the motor resolver, which helped to reduce the motor size.