In this paper, we propose a process calculus, Context-aware System Calculus (CSC), in order to describe context-aware systems that can maintain context models and carry out some operations, including requesting context-aware services, according to the context rules. Besides, a contextaware system can also communicate with other systems or receive context information from context providers by sharing atoms through channels. We present the motivation and syntax of CSC, and demonstrate a case study to encode a contextaware system into CSC.