There are many uncertain factors affecting the result of fault diagnosis of power equipment. Fuzzy set theory is one of the effective tools to solve the problem. This paper proposes the insulation detection of insulator strings based on the realizing process with a fuzzy logical reasoning method

especially the establishment of the fuzzy relation matrix. The selecting process of the input and output fuzzy reasoning parameters and the establishing process of the fuzzy membership function is analyzed. By use of the least square method

the fuzzy relation matrix is derived from the input-output data of a large number of experiment results. With the aid of the established fuzzy relation matrix

the insulation condition of the insulator strings can be detected easily and correctly by using the new developed on-line detection algorithm. The correctness of the proposed method has been verified by experimental results from both the laboratory and 110 kV transmission line.