Security issue of sensor networks is greatly different from that of conventional networks, in terms of its specific requirements, constrained resources of nodes, and variety of network characteristics. The security architecture of sensor networks is proposed, trying to outline a general illustration on this area. The following three topics are discussed: 1) Security primitives such as SKE, MAC and PKC; 2) Various keying mechanisms for the key management issue; 3) Joint considerations on network-related issues including routing, energy and fault tolerance.Other open security issues and further challenges are also addressed.