Monitoring is not simply the collection and enumeration of data points but the discovery of information and knowledge inherent within a system under study. Mobile Surveyor, a model-based monitoring system, provides a new perspective and approach to software monitoring by incorporating data models throughout the monitoring process. This paper presents the construction and usage of ontology in Mobile Surveyor, attempting to capture the conceptual structure that offers semantic terms to model the system and build a generic application-independent monitoring tool. Ontology enables the information and knowledge in Mobile Surveyor to be accessible, sharable, reusable and transformable to the heterogeneous systems and applications that use Mobile Surveyor. The Ontolingua Server with OKBC is used to provide an environment for ontology development and knowledge base query and manipulation. The performance of Mobile Surveyor and the role ontology play in it are demonstrated by using a mobile application as the case study.