The velocity induced eddy current probe has been used to detect cracks in both non-ferromagnetic and ferro-magnetic metals. The simulation and experimental results show that this probe can successfully detect cracks in both cases, but further investigation shows that the underlying principles for inspecting non-ferromagnetic and ferromagnetic metals are actually different. For aluminum plate, the induced eddy current density and the signal amplitude both increases with probe speed, which means the signal is caused by velocity induced eddy current. For the steel plate, however, the signal amplitude is independent of probe speed. Simulation results show that the signal for cracks in steel plate is mainly caused by direct magnetic field perturbation rather than velocity induced eddy current.