This research aims to compare the capabilities of a thesaurus and an ontology for representation of knowledge in Library and Information Science (LIS), in the domain of Indexing and Persian language. The research also has developed a methodology of ontology construction by implementing domain analysis methodology.The present work developed a prototype ontology named ASFAOnt by reengineering a current thesaurus in Persian language named ASFA.The ontology was evaluated by a retrieval test in which the usability and the performance of the ontology were investigated. Usability and performance were compared with ASFA thesaurus. The evaluation was carried out as a controlled experiment in which a group of information searchers, LIS master students, used the thesaurus and ontology to extract three keywords. The evaluative study was followed by a qualitative analysis of the nature of concepts, individual relationships and the textual and visual display so as to provide an understanding of and insight into the quality of the ontology as well as to gather ideas for further development. In order to gather more validated data on users’ performance and satisfaction, a thinking aloud technique was used. Also the experimental retrieval test regarding the usability test was followed by a questionnaire survey, constructed on the basis of the ONTOMETRIC approach.The result of the study indicates that ontology structure is useful and provides valuable inspiration for the user. Fundamental differences of domain knowledge representation between them were then identified.Relationships such as broader term (BT) and narrower term (NT) in the ASFA could support a capacity for reasoning based on generation and specification, assuming the relationships themselves are valid. However, ASFAOnt ontology supports the deduction of conclusion based on domain knowledge described in the ontology. We conclude that ontology can provide a better representation of the domain knowledge and more advanced power of reasoning based on the underlying knowledge representation.