Knowledge acquisition and verification is a critical bottleneck of expert systems. In particular, it is difficult to confirm and maintain the consistency of a large-scale knowledge base. Knowledge verification is the proceess that makes problem solving knowledge complete and consistenst. KNOV (Knowledge Verification System) has been developed for diagnostic applications. KNOV is a meta-system which regards the diagnostic knowledge as an assumption and verifies it by assumption-based reasoning using ATMS. An architecture with the following features is proposed for knowledge verification : (1) assumption-based reasoning with dynamic testing (2) meta-knowledge definition for verification (3) knowledge consistency using ATMS The validity and effectiveness of the ideas for knowledge verification were confirmed by applying KNOV to the diagnosis system of Electric Power Systems and Computer-Center Fault Recovery Systems used in the field. This paper shows a verification example using KNOV for the Electric Power System's diagnosis knowledge base. Last, problems to be solved for practical use of the system are pointed out.