With the growth of the knowledge-based economy, intellectual property right (IPR) is recognized as a key factor to develop and protect the strategic competitiveness and innovation of an enterprise. Patents are used to encourage and protect inventions and to stimulate commercial development complying with legal IPRs. Due to growing number of patents, most patent information management systems lack advanced analytical and decision support methodologies and tools. The purpose of this research is to develop a patent defense system using the combination of ontology engineering and improved overlapping clustering method. The ontology engineering approach analyzes patent claims while structuring and extracting patent characteristics for technical domain schema construction. The research uses the overlapping clustering method to analyze related patents and compare their claim elements based on the patent ontology and infringement rules. The research also studies actual patent litigation cases to demonstrate the effectiveness and applicability of the research solutions.