Believing mutually is an important premise to ensure that cooperation among multiple agents goes smoothly. However, mutual belief among agents is always considered for granted. In this paper, we adapt a method based on the position-exchange principle to reason about mutual belief among agents. To reason about mutual belief formally, we first use a process algebra approach, the pi-calculus, to formalize cooperation plans and agents, and then bind the position-exchange principle into the inference rules. By reasoning about mutual belief among agents, we can judge whether cooperation among agents can go on rationally or not.