Abstract: Domain ontology plays an important role in the system of information extraction. In this paper, after a discuss of the relation of domain ontology and information extraction, the design principles and steps of domain ontology in the system of information extraction are proposed. The domain ontology about monitor is implemeneted in a prototype system of information extraction, and the domain ontology is applied to some tasks of information extraction, including named entity recognition, extraction pattern acquisition, and thematic concept extraction. The application results show that the principles and steps are feasible.