Abstract: The paper is aimed at engine wear fault diagnosis of certain military aircraft during ground testing.Two common oil analysis methods,namely Ferrography analysis and Spectrometric analysis,as well as engine testing monitoring method,are applied to diagnosing the military engine wear faults using Expert System (ES) method.Firstly,according to expert diagnosis experience,the knowledge rules based on IF-THEN format of each method are obtained.Secondly,by wear criterion values of each method,the original symptoms are transformed into character expressions.Finally,the diagnosis results are achieved through forward chaining method.