Abstract: A new digital watermarking algorithm based on wavelet was proposed for images. In the algorithm the two kinds of orthogonal watermarkings, i.e., robustness and reference watermarkings, are added to the wavelet coefficients of carrier images simultaneously after discrete wavelet transform (DWT) and then the images with watermarkings are obtained by inverse discrete wavelet transform (IDWT). The extracted reference watermarkings are used to estimate the attacks on the images so as to estimate further the original robustness watermarkings, since the attacks on both extracted robustness and reference watermarkings are the same. Test results showed the good sensitivity and robustness of the algorithm for the best-known attacks such as noise interference, filtration, image compression/clipping and other image processing actions.