Abstract: A stochastic dynamic model based on the concept of impact ionizationis presented. The stochastic differential equation of microdefect growth is derivedby using the nonequlibrium statistical method, and by solving the equation theformulae of time-to-failure, and breakdown probability are derived. Thus themicromechanism of dielectric breakdown is connected with macroscopic statisticalnature. The results are consistent with the empirical formula and the experiments.