Recent advancement in wireless communication and microelectronics has enabled the design and development of wireless sensor networks with low cost, low energy consumption and high utilization. Many cluster-based wireless sensor network routing protocols have been proposed. However, most of them take little consideration on communication protection, which is important to ensure the network security. In this paper, a lightweight key management approach is presented. Its analysis shows that this approach is an effective solution to the key management of hierarchical clustered wireless sensor networks.