An important part of an expert system is its knowledge base which contains domain dei^endent knowledge. Knowledge base verification is one of the important ¡problems of knowledge acquisition. It is the process of checking that a knowledge base is complete and consistent. An analysis of the rules can detect many potential problems that may exist in a knowledge base. The knowledge base may be incomplete, inconsistent, or even partly erroneous. Those problems unless identified and corrected may cause the inference engine to produce inconsistent results such as conflicting conclusions and sometimes to enter infinite loops. In order to be general, rules with certainty factors are preferred for knowledge representation. This is partly because rules are used in many applications and certainty factors are necessary when knowledge has probabilistic characteristics. Our approacli is to develop a knowledge base verification tool that can be used as a part of a rule-based expert system shell.