A watermark and extracting method and a watermark hardware structure used in an image compression system. A series of data is generated by random variables, such that duplication is avoided. According to characteristics of the human vision system, weighted value of each block is analyzed by classification in the frequency domain. According to the specific weighted value for each block, a watermark intensity is embedded, such that the embedded watermark is not easily observed, while the robustness is retained. Therefore, the original figure is not required for extraction. In addition, the watermark embedding hardware structure can be incorporated in a joint photographic experts group (JPEG) system, so that the image itself contains the watermark data while being intercepted.