An internal ratchet model is established, in which a force-free system is coupled to a subsystem with asymmetric periodic structure, while the latter is in a thermal bath environment. The average velocities of particles driven by a periodic signal in two ways are calculated respectively. In comparison with the result of external ratchet with the same parameters, our results show that the maximum directional current of particles in internal ratchet is larger than that in external ratchet. Under the same temperature, the particles in the internal ratchet are more prone to directional movement.