Owing to the fuzzy and complicated relationship between the insulation faults and the corresponding symptoms of a large generator， it is a tough job to diagnose its insulation faults with an accurate method. For this reason， a fuzzy inference method for the fault diagnosis of generator’s insulation is proposed supported by an SJY-1 RF（radio frequency） monitor. The quantification table for the modifiers of fuzzy quantifiers is given. Also， the membership function of symptoms for the initial supposition and the calculating formula of inference certainty are given. Furthermore， a feasible fault diagnosing expert system based on fuzzy rules is developed. The effectiveness of insulation monitoring and diagnosis for generators with the techniques of RF and fuzzy diagnosis has been proved in practice.