The ability of data concealment is studied adequately. It is proposed that the ability of data concealment can be scaled by use of the degree of edge,the even degree of distribute and luminance. Using the idea of frequency modulation which belongs to communication engineering, frequency modulation watermark which is different from traditional watermark technique is brought up. Constituting baseband signal by selecting the intermediate frequency coefficent of image piece with strong data concealed ability, then, embedding and picking up watermark according to the idea of frequency modulation. This technique can give attention to invisibleness and rubustness, and realize blind examination efficiently.