Periodically poled LiNbO3 was prepared by applying external field. Through a comparison study on switching currents of different sized PPLN, a new method of determining the nucleation time and the velocity of domain tip propagation toward the opposite face of the crystal was proposed. According to this method, a conclusion could be drawn that the nucleation time is 80ns and the tip velocity of switching domain is 0.1667m/s, when the external field is 25.1kV/mm, 50ms square wave pulse.