Abstract: A watermark technique applying to copyright protection of gray-scale images is proposed．To ensure the security of watermark，the watermark image is scrambled before embedding and then expanded to the same size as the original image．The corresponding subband data of the watermark and the original image are combined according to the characteristic of the multiresolution decomposition of the wavelet transform．Finally，when detecting the presence of watermark，a new method using Fnorm to implement similarity measurement is presented．Experimental results indicate the feasibility and effectiveness of the proposed algorithm．