Abstract: The method of applying an object oriented knowledge representation in an expert systems developing tool is introduced. A knowledge representation language is designed and implemented based on the formalized definition of object and the specification of our expert system developing tool. Because of the property of encapsulation and inheritance of the object oriented technology, the system performances, such as the code reusability, message transformation and the implementation of graphic interface, are improved. The object oriented knowledge representation language has reinforced the representing ability of structured knowledge in the expert systems tool. The combination of object oriented method and knowledge representation can strengthen the practicability of expert systems.