Ontology is applied to agent modeling and reasoning is one approach for research on agents. Modeling virtual battlefield based-on ontology is proposed to support computer generated forces base-on agent technology. Researches of ontology being applied to military modeling and environment modeling are analyzed. The virtual battlefield ontology model is presented. The model includes four domains: nature environment, physical objects, situation and operation plan. Models of the four domain ontology are built. The model provides consistent semantic representation and common understanding for knowledge sharing in military simulation and computer generated forces.