Abstract: In an aiming system it is hard to show dynamic behaviors in fault diagnosis. Fuzzy Petri net is applied to the method of modeling fault transmission. Fuzzy production rules are introduced into knowledge representation, and the fault diagnosis method using fuzzy Petri net and its reasoning algorithm are discussed. Using this method in fault diagnosis in an aiming system, the result demonstrates that fuzzy Petri net has an easy structure, a good expression of faults and effective capability of fuzzy reasoning. Fuzzy Petri net is a useful method in fault diagnosis.