This article presents a new approach in order to index a Web site. It uses ontologies and natural language techniques for information retrieval on the Internet. The main goal is to build a structured index of the Web site. This structure is given by a terminology oriented ontology of a domain which is chosen a priori according to the content of the Web site. First, the indexing process uses improved natural language techniques to extract well-formed terms taking into account HTML markers. Second, the use of a thesaurus allows us to associate candidate concepts with each term. It makes it possible to reason at a conceptual level. Next, for each candidate concept, its capacity to represent the page is evaluated by determining its level of representativeness of the page. Then, the structured index itself is built. To each concept of the ontology are attached the pages of the Web site in which they are found. Finally, a number of indicators make it possible to evaluate the indexing process of the Web site by the suggested ontology.