Abstract: The problem of fault diagnosis of a tree-type power network with a short-circuit or an open-circuit and a conrac resistance of arbitrary value between lines is studied. Based on nullor substitution, the method presented here is suitable for fault locating and contact resistance evaluating. Principles of diagnosis are discussed, diagcosis equations are established, and diagnosis algorithms are presented. Finally, examples for diagnosing a five-branches tree-type power network with a shore-circuit or an open-circuit art given.