A resistance device applied to relative rotations between a flywheel and an axis includes an inner stator, an outer rotor, a conductive wire and a magneto-rheological fluid. The inner stator is fixedly joined with the axis and includes an accommodating space surrounding the axis at a position away from the axis. The outer rotor, fixedly joined with the flywheel, encloses and rotates relative to the inner stator. An accommodating gap is formed between the outer rotor and the inner stator at a position away from the axis. The conductive wire is wound in the accommodating space, and generates a magnetic line passing the accommodating gap when applied by an electric current. The magneto-rheological fluid is filled in the accommodating gap. Thus, the outer rotor is disposed at the outer most region of the resistance device to increase the braking torque, and the magneto-rheological fluid is away from the axis to increase the braking moment.