Based on the physical mechanism of gate oxide TDDB, a percolation model for gate oxide degradation was brought forward, in which the occurrence and build up of deep energy-level defects, such as E′ center and oxygen vacancy were considered to be the right cause of oxide breakdown. It was pointed out that, during TDDB the stressed defects were produced in the oxide, which form local states in oxide forbidden gap. And the volume of these local states is directly proportional to the external electric field, especially when the field strength is high enough. With the by-pass of stressing time, the concentration of defects in the oxide grew continually. As an effect, the distance between neighboring local states become shorter. So, hopping or tunneling probabilities of electrons between these local states increase swiftly. As a result, a conduction path will form when the distance between neighboring local states reaches a critical value. At the same time, in terms of energy band theory, an extended energy level will form in the oxide forbidden gap. Accompanied by a rapid increase of SILC, the gate oxide undergoes breakdown.