Grid has emerged as a new paradigm for integration within dynamic virtual enterprises. Given a service-oriented Grid environment, more complex, value-added sophisticated services and applications can be built via service composition. In this paper, we discuss Grid services composition by leveraging Semantic Web services standards and technology, especially OWL-S. The characterization of service composeability is presented in the context of OWL-S. The techniques for constructing the Web/Grid service composer are described, and the focus is placed on composition engine and OWL-S Matcher.