Knowledge acquisition, maintenance of knowledge base, and validation and verification of knowledge are the addressed bottlenecks of building successful knowledge based systems. Along with the increment of interesting in the knowledge based systems, the organization needs to develop a new one although it has a similar one. This causes several serious problems including knowledge redundancy and maintenance of knowledge base. This paper present three models of the reusable knowledge base which might be the solution to the above problem. Three models are : 1) multiple knowledge bases for a single AI application, 2) multiple knowledge bases for multiple AI applications, 3) a single knowledge base for multiple AI applications. A new approach to build such a reusable knowledge base in a homogeneous environment is presented. Our model combines the essential object-oriented techniques with rules in a consistent manner. Important aspects of applying object-oriented techniques to AI are discussed (inheritance, encapsulation, message passing), and some potential problems in building an AI application (decomposition technique of knowledge, search time, and heterogeneous environment) are pointed out. The models of a reusable knowledge base provide several amenities : 1) reduce the knowledge redundancy, 2) reduce the effort of maintenance of the knowledge base, 3) reuse the resource of the multiple domain knowledge bases, 4) reduce the development time.