An internal magnetic resistance system for use with a fitness device includes an axle, inertia wheel, transmission wheel, magnetic permeable ring and electromagnet. The inertia wheel is disposed at the axle and has a protruding ring portion. The transmission wheel connects to the inertia wheel and transfers an external force to the inertia wheel to cause the inertia wheel to rotate. An outer circumferential surface of the magnetic permeable ring is fixedly disposed on an inner circumferential surface of the protruding ring portion such that the magnetic permeable ring rotates together with the inertia wheel. The electromagnet is disposed in the protruding ring portion and surrounded by the magnetic permeable ring. The electromagnet has a support fixedly disposed at the axle and iron cores disposed at the support. Each said iron core is surrounded by a coil and separated from the magnetic permeable ring by a gap.