Abstract: To address the problem associated with the minimum diagnosis bases (MDBs) of discrete event systems (DESs), we present an algorithm that is based on the event set tree. First, we build a new type of test automaton, based on the diagnoser, and obtain the necessary and sufficient conditions for DESs to be able to diagnose faults. We propose an algorithm for computing the MDB by constructing an event set tree, and analyze the complexity of the algorithm. Finally, we evaluate the performance of the algorithm by its application to the selection of key networks in network systems. Compared with the results of existing algorithms, the proposed algorithm is not only less complex, but it can also be used for both online and offline diagnoses.