Service Oriented Opportunistic Sensor Networks (TOppS) project at the Swedish Defense Research Agency aims to offer user numbers of services from sensor networks. As non- technicians, end-user of TOppS would require a user-friendly GUI with the ability of manipulating the gathered web services to create new service which can meet their own demands. In this work, we designed and implemented a Web-based graphical workspace prototype called Service Composition Workspace (SCW) using Semantic Web technologies such as includes Web Ontology Language (OWL), Ontology Web Language for Services (OWL-S), etc. SCW offers not only the capability of dynamically composing Web services, but also the ability of outputting "machine-understandable" data, which can be directly processed by computer system without human intervention. The workspace also provides a drag & drop user interface for end-users to easily create and modify their own system configuration on-the-fly. The effectiveness and functionality of proposed model is evaluated by vast test cases.