This paper proposes a method of Discrete Event System (DES) to deal with the fault section estimation problems in the bulk power system. Our approach of failure diagnosis is developing a discrete-event model of the system to be diagnosed followed by construction of the diagnoser. A clear framework is built based on the model description language of Discrete Event System (DES). Moreover, a mechanism of reasoning based on model state trajectories is introduced during the diagnosis procedure. Finally, an example is verified by the proposed method in actual power network of province Jiang Su in China. (6 pages)