The nonlinear a.c. response of the flux motion in type II superconductors is studied using a single flux motion model. We have performed a numerical simulation and evaluated the a.c. response against frequency. Three cases of applied current are considered: a single-frequency a.c. current, an a.c. current of two frequencies and a d.c. plus an a.c. current. We find that, when the Lorentz driving force is greater than the pinning force, there exists a crossover frequency below which a strong nonlinear a.c. response is exhibited. We also find that the value of this crossover frequency has a linear relation with the ratio of the Lorentz and the pinning forces.