Abstract: The nodes of wireless sensor network are very limited in its processing power, storage capacity and communications capability, so it is usually to be considered unsuitable for the use of public key cryptography technology. However, in recent years many research results indicated that some optimized public-key algorithm can provide much more security for wireless sensor networks (WSN). The identity-based public system simplifies the certificates management which is cumbersome in certificate based public key system. According to the features of WSN, the security model of identity-based public-key system is modified. A new identity-based signature (IBS) system is presented and proved secure in the new security model without random oracle. Storage requirement in previous IBS systems is directly related to message length, but it is constant in our new system, so the new system is more suitable for wireless sensor networks.