This paper proposes a new gas turbine power generation system called the "motor-assisted gas turbine" (MAGT). The MAGT is composed of a dual-shaft gas turbine and an inverter-fed motor. The conventional dual-shaft gas turbine has two shafts that rotate at different speeds: a power turbine shaft and a compressor shaft. The power turbine shaft is connected to a synchronous generator as usual. In the MAGT, the new idea is that the compressor shaft is coupled to an inverter-fed motor. By assisting the compressor with this motor, the MAGT is able to increase the power output of the gas turbine when the intake air temperature is high. In addition, the MAGT can be used as an energy storage device like a flywheel. The inertia energy of the compressor can be charged and discharged by varying the rotation speed with the motor directly connected to the compressor. This paper also presents a method for designing motors that are suitable for MAGTs. The interior permanent-magnet synchronous motors were studied by using a joint optimization method of stress and electromagnetic analysis for megawatt-class high-speed applications.