The vehicle power generation system includes: a power generation unit, a power supply unit, a clutch mechanism, and a control unit. The power generation unit includes generator, power input shaft, and a power output unit. The clutch mechanism includes a speed increasing unit and a motor controller for attaching the speed increasing unit to the crankshaft or detaching the speed increasing unit from the crankshaft. The control unit is electrically connected to the motor controller, and includes a power generation condition determining unit.