Abstract: In this paper, some serious shortcomings of existing expert systems are described The importance of fuzzy expert systems is pointed out. Based on test-score semantics, the canonical method for fuzzy propositions and the organization of fuzzy knowledge base are discussed, and the inference mechanisms of fuzzy expert systems, i.e the fuzzy logic inference for unqualified propositions, the intersection/product syllogism, the consequent conjunction syllogism and the inference of propositional chain for quantified propositions, are investigated.