A rotational kinetic energy output device includes a housing component, a rotational component and a magnetism generating component. The rotational component includes a rotational body pivotally connected to the housing component, and the rotational body has at least one magnetic element. The magnetism generating component includes a plurality of magnetizers and coils wound on the magnetizers, the magnetizers are disposed in the housing component and surround the rotational body. Each of the magnetizers has a first magnetic arm and a second magnetic arm extended respectively toward the rotational body, and the rotational body is disposed between the first magnetic arm and the second magnetic arm. Thereby, the rotational kinetic energy output device allows a force to be balancedly exerted on the rotational body and a thin shape is also achieved.