Security is an important issue in sensor networks. Many applications in military, distributed information gathering etc., demand for Secure Group Communication (SGC) in sensor networks. The SGC requires common network-wide group key for confidentiality of control messages and data reports. The group key should be updated when a node is compromised. In this paper we propose a new key management scheme for group key computation and distribution which is based on tree structure. The proposed scheme minimizes storage as well as communication and computation cost of end user (i.e., sensor nodes). The complex encryption/decryption operations used to distribute new group key whenever a node is compromised are replaced by one way hash functions and simple XOR operations.