An attenuation reduction grounding structure of differential-mode signal transmission lines of a flexible circuit board includes a flexible substrate on which at least one pair of differential-mode signal lines, at least one grounding line, a covering insulation layer, and a thin metal foil layer are formed. At least one via hole extends through the thin metal foil layer and the covering insulation layer and corresponds to a conductive contact zone of the grounding line. The via hole is filled with a conductive paste layer to electrically connect the thin metal foil layer to the conductive contact zone of the grounding line to provide an excellent grounding arrangement. The thin metal foil layer includes a plurality of openings formed at locations corresponding to top angles of the differential-mode signal lines.