Sharing and enriching of documents is expected and is made possible nowadays with tools enabling users to perform different kinds of annotations. We propose an Ontology-based approach to automate the semantic annotation of texts; Ontologies are represented in OWL (Web Ontology Language). OWL is supported by Semantic Web tools such as Racer for reasoning purpose and Jena. The tool for automatic semantic annotation supporting word-based and stem-based pre-indexing techniques is presented and its evaluation is made on three medical corpora both in English and French (brain disease area, cardiology and OHSUMED collection). The evaluation shows difference in the results obtained according to the pre-indixng mode used.