Users and systems requirements are fundamental for software development and maintenance. However, for most of existing systems, you may only find design documents without requirement specification. This paper presents an ontology-based reengineering approach to recovering requirements from existing systems. The proposed approach consists of three main parts: ontology development, ontology mapping and derivation of the requirements. Domain ontology is used to model domain specific requirements and program ontology is used to present system structure and behaviour. The algorithm of ontology mapping is developed to match domain ontology and program ontology for requirement recovery. A case study of Point of Sale Terminal (POST) system is used to illustrate the approach. Conclusions are drawn and further research directions are advocated.