Abstract: A method for stabilizing breakdown voltages of floating guard ring, applicable to a high power device, is provided. The high power device has a semiconductor substrate layer, and at least one floating guard ring is formed at its termination. The method includes sequentially providing a pad oxide layer and barrier layer on an upper surface of the high power device to expose the floating guard ring, and then performing an ion implantation step. After removing the pad oxide layer and barrier layer, grow a field oxide layer, such that a defect layer is formed underneath. By employing the formed defect layer, the present invention achieves to control an interface potential level between the field oxide layer and the semiconductor substrate layer fixed at a certain potential value, without being affected by charges in the oxide layer or metal across over it, thereby stabilizing breakdown voltages of floating guard ring.