A fast dot/dash line detection method suitable for large scale binary document images is proposed. The method works by reducing the number of scanned pixels used for the detection process. In the new method, pixels in vertical image layers with only a constant spacing are scanned. By using this technique, the computational time can be reduced because some of the uninteresting objects in the image can easily be omitted in the scanning stage. The new method is faster than the conventional method not only due to its scanning method but it also due to the simple process used for detecting dot/dash lines. A dot/dash line is detected by selecting a small defined image domain from the large scale image. We evaluated the new method against conventional methods on appropriate document images and found an improved processing time without any significant loss of line detection ability.