Abstract—An improved S-MAC protocol is proposed based on data priority and energy priority. The priority of data is identified according to a correlative strategy at the source node in WSN. Both data and its priority are encapsulated into a frame and transmitted in network. When forwarding a data frame, the node adjusts the contention window dynamically according to the data priority and energy priority to make sure that the high priority data frame are forwarded prior and the low energy nodes forward data frame prior when their data priority are same. The simulation results show that the improved protocol not only can achieve the efficient transmission for high-priority data, but also reduce the dead nodes to almost 30%.