The Unified Modeling Language (UML) is the de-facto standard modeling language for the development of software with broad ranges of applications. It supports for modeling a software at different stages during its development: requirement analysis, design and implementation. The models to produce for an object-oriented requirement analysis are a conceptual class model and a use-case model. This paper proposes a method to combine these two models by using a classic transition system. Then we can reason about and refine such systems with well established methods and tools.