In this paper, we propose a Core Grid Ontology (CGO) that defines fundamental Grid-specific concepts, and the relationships between them. One of the key goals is to make this Core Grid Ontology general enough and easily extensible to be used by different Grid architectures or Grid middleware, so that the CGO can provide a common basis for representing Grid knowledge about Grid systems, including Grid resources, Grid middleware, services, applications, and Grid users. The Core Grid Ontology is designed and developed based on a general model ofGrid infrastructures, and described in the Web Ontology Language OWL. Such an ontology can play an important role in building Gridrelated Knowledge bases and in supporting the realization of the Semantic Grid.