Over the past decade, improving healthcare quality and safety through patient safety event reporting systems has drawn much attention. Unfortunately, such systems are suffering from low data quality, inefficient data entry and ineffective information retrieval. For improving the systems, we develop a semantic web ontology based on the WHO International Classification for Patient Safety (ICPS) and AHRQ Common Formats for patient safety event reporting. The ontology holds potential in enhancing knowledge management and information retrieval, as well as providing flexible data entry and case analysis for both reporters and reviewers of patient safety events. In this paper, we detailed our efforts in data acquisition, transformation, implementation and initial evaluation of the ontology.