Abstract: Failure diagnosis of discrete event systems (DESs) has received considerable attention in recent years. The failure diagnosis for fuzzy discrete event systems (FDESs) modeled by fuzzy automata is investigated. First, for two kinds of failure patterns: S type pattern and T type pattern, the notions of diagnosability for S type pattern and for T type pattern of FDESs are respectively formalized. Then by constructing S type pattern diagnoser and T type pattern diagnoser, the necessary and sufficient conditions for the diagnosability of S type pattern and T type pattern of FDESs are respectively proposed, and the pattern-based failure diagnosis of FDESs is realized.