Context-awareness plays an important role in developing flexible and adaptive systems. However, it is not easy to design and implement such a context-aware system, because its system configuration can be dynamically changed. This paper proposes UML4COP, a UML-based design method for COP (Context-Oriented Programming). UML4COP is a DSML (Domain-Specific Modeling Language) for designing context-aware systems. In UML4COP, each context is modeled separately and a system design model at a certain period of time is composed by merging associated contexts.