Abstract: Current group proxy signature schemes are not proved for their security in the complete security model for group proxy signature, and do not involve the concept of identity. In this paper, we show a complete security model for group proxy signature based on Boldyreva's security model. And a new identity-based group proxy signature scheme is proposed in the standard model based on the Paterson's scheme. In our security model for group proxy signature, the new scheme is proved to have the existential identity-based group proxy signature unforgerability under an adaptive chosen message attack, and have a security reduction to CDH problem. Compared with other group proxy signature schemes based on public key cryptosystem in the standard model, the new scheme is more secure and involves the concept of identity.