AbstractThe power transformer is a major apparatus in a power system, and its correct functioning is vital to system operations. Failure of one power transformer may cause long interruptions in supply, costly repairs and loss revenue. The analysis of oil‐immersed gases as a useful and effective method can prevent main faults through diagnosing internal faults of power transformers and proposing preventive operations. Usually the relation between faults and the reason of their existence in power equipments is complicated. Therefore, fuzzy logic method is proper option in the diagnosis of transformers internal faults. In this paper, three fuzzy methods are used for specifying the internal faults of transformer through the ratio method of oil‐immersed gases. Finally, comparing with commonly used methods, the proposed methods found effective enough. Copyright © 2009 John Wiley & Sons, Ltd.