Digital watermarking has been presented in recent literatures as a technique to protect the copyright of multimedia data. But most of these literatures focus on the algorithms of embedding one-dimensional watermarks or two-dimensional binary digital watermarks. In this paper, a method of embedding a gray-level digital watermark into an image is proposed. By stack filter's threshold decomposition technique, a gray-level digital watermark is decompounded into a series of binary digital images for implementing multiple watermarking. The experimental results show that the proposed techniques are successfully tolerant to reasonable image processing operations and the lossy compression techniques such as Joint Photographic Experts Group (JPEG).