This paper is towards the development of a methodology for object-oriented software development. The intention is to support effective use of a formal model for specifying and reasoning during the requirements analysis and design of a software development process. The overall purpose is to enhance the application of the Unified Modelling Language (UML) with a formal semantics in the Rational Unified Software Development Process (RUP). The semantic framework defines the meaning of some UML submodels. It identifies both the static and dynamic relationships among these submodels. Thus, the focus of this paper is the development of a semantic model to consistently combine a use-case model and a conceptual class diagram to form a system specification.