Abstract: A significant feature of the new generation knowledge base is the introduction of object oriented paradigm.An object oriented knowledge base system named CAOBS/V1 2 is introduced in the paper.Some key issues such as object oriented knowledge representation,inference and visual query mechanism are studied intensively.In CAOBS/V1 2,a case based reasoner collaborates with a rule based reasoner on problem solving.And VKQL is a visual knowledge query language based on two dimension query graph.The major components of the system are described and the metaobject guided case based reasoning strategy related to diesel fault diagnosis is discussed in detail.Finally,this system is compared with other similar systems in the world.