Since the nodes in wireless sensor networks operate on limited power, it is an important issue to design efficient data gathering scheme that can save energy while meeting the needs of application. In this paper, we propose an energy-efficient cluster-based data gathering protocol (ECDGP). ECDGP uses a new clustering algorithm based on belief degree for clusterhead election. Furthermore, ECDGP selects the active nodes according to the demand of network coverage. By controlling the number of active nodes in a cluster, ECDGP reduces energy consumption and prolongs the network lifetime. Simulation results show that ECDGP significantly outperforms the traditional clustering routing algorithms in terms of energy consumption and network lifetime.