Because the existing robust watermarking algorithms cannot involve the research of robustness against dual print-and-scan process, a robust zero-watermarking algorithm against dual print-and-scan process was proposed. The algorithm was based on the stability of the numerical relationship between each block′s direct coefficient and the average of direct coefficients form all blocks in discrete cosine transformation domain. The image was divided into non-overlapping blocks, and each block was transformed with discrete cosine transformation. Zero-watermark sequence was produced by judging the numerical relationship between each block′s direct coefficient and the average of direct coefficients form all blocks. Some tests were carried out including its robustness against single print-and-scan process, conventional signal processing after single print-and-scan process, dual print-and-scan process, and conventional signal processing after dual print-and-scan process. The results show that the proposed algorithm has excellent robustness towards those above attacks.