Wireless sensor and actor networks (WSANs) are composed of sensors and actors to perform distributed sensing and acting tasks. To accomplish effective sensing and acting tasks, efficient coordination mechanisms of sensor-sensor, sensor-actor and actoractor are required. In this paper, a novel three-level coordination model for WSANs is proposed, based on a hierarchical geographical clustering paradigm in which cluster formation is formed by dividing the action area into fixed zones to form a virtual grid, so that the working area and workload can be optimally split up into different actors. In this model, sensors are stationary and location-aware, whereas actors may change their locations dynamically. In different levels, sensor-sensor, sensor-actor, actor-actor coordination mechanisms are addressed respectively. The functions of each level are loose coupling with each other so that many different routing and communication protocols can be adopted. Case study shows our model is effective.