Context-aware computing is an important aspect of the pervasive computing environment and its various dynamic context information brings new challenges to access control systems. In this paper a new access control model, relation based access control (RelBAC), is provided for context-aware environment with a domain specific Description Logic to formalize the model. The novelty of RelBAC is that permissions are formalized as binary relations between subjects and objects which could evolve with the dynamic contexts. The expressive power of RelBAC is illustrated in a case study of a project meeting event.