This paper shows that high-speed starter/generator system is more efficient for gas turbine engine for mobile auxiliary power unit. The system is rated at 25kW, 325Vdc, 60krpm .The system also provides 4kw to start the 100kW engine. The system consists of a high speed machine directly coupled to the gas turbine engine, a power control unit (PCU), and an electronics controller. The PCU is consist of boost converter that boost from 24V(Battery of Vehicle) to 235V for driving high-speed motor, inverter drive PMSM (Permanent Magnet Synchronous Motor), and buck converter drop the voltage to 28V. For PMSM driving the system applied SVPWM (Space Vector Pulse Width Modulation), sensorless algorithm. And then, to supply optimized power, "Constant Power Control Algorithm" is applied.(a) Structure of 3 shafts 1 CONCLUSION This paper has described the design objectives, hardware, control algorithm and test results for a high-speed permanent magnet motor/generator system. This paper also shows possibility of high-speed motor/generator system for gas turbine engine starting, power generation. And mobile APU is come true due to boost converter, optimization of starter system is possible using the constant power control algorithm.Future work will be made toward more compact and effective starter/generating system for other application.