Service-Oriented Architecture has the major advantage of enabling rapid composition of distributed applications from various services, and has become increasingly popular for many large-scale service-based systems in various application areas, including scientific collaboration, e-business, health care, military, and homeland security. Situation awareness (SAW) is the capability of the entities in a service-based system to be aware of the situation changes and automatically adapt themselves to such changes to satisfy user requirements, including security and privacy. The continuing evolutions of the entities and environment makes SAW one of the most desired features to support dynamic adaptive computing in service-based systems. In this paper, the relationship between contexts/situations and services in situation-aware service-based systems is identified and an extension of OWL-S with situation ontology, called SAW-OWL-S, incorporates SAW in service specifications is presented. An approach to generating service specifications for situation-aware service-based systems using SAW-OWL-S and the system diagram of situation-aware service-based systems using SAW-OWL-S are presented.