Ontology-based information extraction is considered as an effective method to improve the performance of information extraction (IE) systems. In order to build a better IE system using ontology-based technique, the two challenges should be most taken into account are semantic elements extraction, and ontology enhancement. Focusing on the two key points above, we propose an ontology-based information extraction system and its application information retrieval system in the health care domain. In ontology-based information extraction system, we propose a non part-of-speech based method for the semantic elements extraction and the ontology enhancement to overcome the above challenges. Experiments and Results show that all functions work well in the whole system. Many kinds of semantic elements can be extracted by the proposed extraction functions regardless parts-of-speech of words. High ratio of new semantic elements is detected for the ontology and database enrichment that yield very good results in information retrieval. The system is implemented in Vietnamese.