In this paper, the fuzzy diagnosis for rotor's faults is dealt with in two steps. Firstly, to determine relatively the kinds of faults on rotors, the equation of fuzzy relations is estabilished in terms of synthesis judgement, then it is solved by the least square method. Secondly, to determine the gradation of a certain fault on the rotor, the fuzzy method of pattern recognition is employed according to the nearest neighbor classification rule.