Power transformer is a complex system. It is difficult to gain complete experiment data in actual fault diagnosis. To solve the problem, the paper proposed a new method of fault diagnosis of power transformer based on rough set. The implementing steps of the method are as follows: analyzing collected history fault data of power transformer to determine condition attribute set and decision attribute set; deducting condition attribute set to remove redundant information and extract key information, so as to gain relative rule set; making fault diagnosis for power transformer by use of the rule set. The case analysis proved validity and effectiveness of the method.