A flexible substrate includes a base layer, a metallic layer, a solder mask layer and an identifying code, the metallic layer is disposed at a first surface of the base layer, the metallic layer comprises a plurality of traces and at least one bottom block used for defining marked position, wherein the traces and the at least one bottom block are covered with the solder mask layer, wherein above the perpendicular direction of the at least one bottom block of the metallic layer, a pre-marked area is defined on an exposing surface of the solder mask layer and by an outlined edge of the at least one bottom block, and the identifying code is formed within the pre-marked area of the solder mask layer.