In this paper, we propose a new document classification model which utilizes background knowledge gathered by ontologies for document representation. A document is represented using a set of ontology concepts that are acquired by exact matching technique and through identification and extraction of new terms which can be semantically related to these concepts. In addition, a new concept weighting scheme composed of concept relevance and importance is employed by the model to compute weight of concepts. We conducted experiments to test the model and the obtained results showed that a considerable improvement of classification performance is achieved by using our proposed model.