Attribute-based signature and proxy signature are both very useful in many real-world applications. In this paper, we combine the special features of both signatures and propose an attribute-based proxy signature scheme, where the original signer, who possesses a set of attributes, can delegate his/her signing right to a designated proxy signer. By verifying the signature, a verifier can be convinced that the signature is generated by the proxy signer who has obtained the delegation from a legitimate signer whose attributes satisfy a predicate. However, the verifier cannot tell from the signature who is the original signer. We provide the formal definition and adversarial models for attribute-based proxy signature, and an efficient scheme that supports threshold predicates.