A flexible circuit board includes a flexible light-permeable carrier, a circuit layer, a mark and a stiffener. The circuit layer and the mark are located on a top surface of the flexible light-permeable carrier. A predetermined area and a stiffener mounting area corresponding to each other are defined on the top surface and a bottom surface of the flexible light-permeable carrier, respectively. The mark is opaque to create a shadow mark having a longitudinal reference side and a lateral reference side on the bottom surface. The stiffener is adhered to the stiffener mounting area defined on the bottom surface by aligning with the longitudinal reference side and the lateral reference side of the shadow mark.