Large amounts of digital contents can be easily duplicated by personal computers. Illegal copies have become serious problem in recent years. Digital watermark technology is a solution of this task, and various kind of digital watermark techniques have been proposed. In general, there are two types of digital image watermarking schemes, that is to say, one is the scheme based on spatial domain and another is the scheme based on frequency domain. In the latter scheme, wavelet transform or DCT can be used and the watermark is embedded into their coefficients. This scheme makes watermark spread out into the whole image, and therefore, gives robust behavior to the frequency domain based attack like JPEG algorithm. However, it is difficult to extract the watermark from the partial image trimmed by cutting off the original one, because the starting position of the DCT block and the cut position are not usually same. The starting position of the DCT block has to be detected in order to extract the embedded watermark from the partial image. In this paper, we propose the detection method for DCT block boundary of original image from the trimmed partial image. Embedded watermark in the DCT coefficients can be extracted by detecting the DCT block boundary of the original image, with our proposed method.