Abstract: A digital watemark ing algorithm based on fractional Fourier transform is proposed in this paper First the original inage is divided into blocks then fractional Fourier transform is performed on every blocked inage finally the robust watermarking and the fragile watermarking are embedded to the fractional Fourier domain of the blocked inage respectively. The experiment results indicate that the algorithm can satisfy the invisibility security and robustness of the watermarking better Most importantly the tampered area can be accurately located.