Abstract: A method of fault assessment/diagnosis that combines fuzzy logic with expert system is put forward to the uncertainty problems to be solved in equipment fault diagnosis. It is done in the way the rules originated from fuzziness are introduced in fuzzy expert system to express knowledge obtained with fuzzy matching procedure adopted in the uncertainty reasoning process to implement the diagnosis based on knowledge through a strategy to resolve conflicts with the confidence level taken as criterion. The functional components and implementation of the fuzzy expert system are introduced, which has been applied to the fault diagnosis using gas chromatography for power transformers. The percentage of accuracy of the method is up to 95.7% as proved by the practical diagnosis done in site near 100 times, verifying the fact that applying the fuzzy expert system to the diagnosis for power transformers is applicable.