In the wireless sensor networks (WSNs), severe energy constraint necessitates energy-efficient protocols to fulfill application objectives. In this paper, we propose a novel cross-layer energy-efficient protocol-CLEEP, which adopts cross-layer strategy that considers physical layer, MAC layer, and network layer jointly. In the physical layer, we first coordinate the transmission power between two nodes and maintain the nodes' neighbor tables periodically to save the transmission energy. Then we construct the optimal routing path by exploiting the transmission power and neighbor tables of the physical layer, which minimize the total energy consumption. Finally, MAC layer make use of the routing information to determine the node's duty-cycle, in order to prolong the node's sleep time. Simulation reveals that CLEEP is energy-efficient and able to achieve significant performance improvement as well.