Abstract: A comparative study on the theory and method of building up a knowledge base for oil analysis fault diagnosis is proposed and discussed. Through on site tests,investigations as well as collection of knowledge and experience of maintheance experts,followed by the conceptualiation, formalization and design of the expert knowledge, a knowledge base containing more than 300 rules is established. Examination in field damnstrathes that the system OAFDES has high annoy of prediction and good applicability.