In order to overcome the limitation of the watermarked image quality degradation due to the some changes of the original image data when embedding watermark in the traditional digital watermarking schemes, a robust adaptive zero-watermarking algorithm based on the combined transforms of the Discrete Wavelet Transform (DWT) and Discrete Cosine Transform (DCT) is proposed. First, the original digital image is decomposed into the appropriate levels of DWT, and the DCT is applied to the obtained wavelet approximation sub-image, and then the DCT AC coefficients are selected adaptively in orders according to the size of the embedded digital watermark and the coefficient difference value. Finally, after the combination of the original binary character watermark and the binary key image which is obtained from comparing the selected DCT AC coefficients, the embedding of zero-watermark is realized by registering to the center of authentication. The results demonstrate that the proposed zero-watermarking algorithm not only ensures the watermarked image quality without distortion, but also has very good robustness against the conventional treatments (for example: noise addition, filtering, JPEG compression, rotation) , and is better than the some reported zero-watermarking algorithms.