In Wireless Sensor networks clustering algorithm is an effective technology to save sensor nodes energy and extend network lifetime, but most of these algorithms are often designed for homogeneous sensor networks, which is not realistic and practical. In this paper, we propose an improved clustering protocol BEEC (Balanced and Energy-Efficient Clustering algorithm) for heterogeneous wireless sensor networks. In BEEC, it optimizes the threshold for selecting cluster heads and uses coverage radius to balance the distribution of cluster heads. It also introduces a novel approach to choose cluster head for cluster members so as to take fully advantage of energy heterogeneity. Simulation results show that this improved clustering algorithm can effectively prolong network lifetime and enlarge network throughput when compared with other existing clustering protocols in heterogeneous environments.