Transformer deficiencies usually take many years to gestate before developing into a problem or failure. Thus the loss induced by problems or failures could be decreased if they are diagnosed earlier and a maintenance action is adopted duly. Many methods had been developed to finish this purpose, but the diagnosis results were still unsatisfied. In this paper, a case-based reasoning method is utilized to diagnose the incipient fault of power transformer. At first, the principle of case-based reasoning is introduced. And then, the re-division of transformer fault is carried out in order to give more specific and useful diagnosis conclusion. After that, a case-based diagnosis method is constructed, and detail diagnosis course is presented. This diagnosis result could give more definite fault position, thus it will give better instruction for the transformer maintenance. At last, the application results are illustrated, and it shows that this purposed method improves the reliability and practicality