Wireless sensor networks(WSN) are randomly deployed in adversary or dangerous environments so that the security of WSN is extremely important. It is necessary to use encryption system to provide security. W propose an efficient key management scheme for WSN. The main purpose of this scheme is to solve secure communications and key management. The proposed scheme allows to evict the compromised node from the network and employs a multi-tier network architecture in which secure sessions are established only between sensor nodes and gateways. The proposed scheme is based on the theory of combinatorial optimization and provides an approach to maintain security while members have changed in groups.