Cancer nanotechnology research data are diverse. Ontologies that provide a unifying knowledge framework for annotation of data are necessary to facilitate the sharing and semantic integration of data for advancing the research via informatics methods. In this work, we report the development of NanoParticle Ontology (NPO) to support the terminological and informatics needs of cancer nanotechnology. The NPO is developed within the framework of the Basic Formal Ontology (BFO) using well-defined principles, and implemented in the Ontology Web Language (OWL). The NPO currently represents entities related to physical, chemical and functional descriptions of nanoparticles that are formulated and tested for applications in cancer diagnostics and therapeutics. Public releases of the NPO are available through the BioPortal web site, maintained by the National Center for Biomedical Ontology. Expansion of the scope and application of the NPO will depend on the needs of and feedback from the user community, and its adoption in nanoparticle database applications. As the NPO continues to grow, it will require a governance structure and well-organized community effort for the maintenance, review and development of the NPO.