Sensor networks are widely used in a variety of commercial and military applications. As a fundamental requirement for providing security functionality in sensor networks, key management plays an essential role in authentication and encryption. In this paper, we propose a group-based dynamic key management scheme in wireless sensor networks. In our scheme, there is no requirement for such infrastructure as base stations and cluster heads and the dynamic key update feature ensures the security of the network without tampering the compromised sensor nodes. We also analyze the connectivity and security of the proposed scheme.