A Digital Library (DL), like any other library management system, supports documents related to different subject areas. These documents may be collected in one place or may be distributed in many different repositories, seamlessly integrated to offer a composite view to a user. The metadata structure provided for the bibliographic search makes the source of a document transparent to any user of the library. In a library, related materials are usually referred by a common index. The most common classification systems like Dewey Decimal Classification(DDC) or LCC provide a strict classification of artefacts. With the help of ontology, concepts of the digital library can be arranged according to the knowledge relationship of them. Hence, the recent development of the Digital Library Research recommends that Ontology can improve the indexing and cataloging of the digital library. According to the digital library community, polyhierarchic structure should be present in digital library ontology. However, existing access control mechanisms for semantic web do not support polyhierarchic structure.