Key escrow is an inherent disadvantage for traditional ID-based cryptosystem, i.e. , the dishonest private key generator (PKG) can forge the signature of any user, meanwhile, the user can deny the signature actually signed by him/herself. To avoid the keyescrow problem, an ID-based signature scheme was presented without trusted PKG. The exact proof of security was presented to demonstrate that our scheme is secure against existential forgery on adaptively chosen message and ID attacks assuming the complexity of computational Diffie-Hellman(CDH) problem. Compared with other signature schemes, the proposed scheme is more efficient.