A flexible printed circuit board (FPCB) includes a signal layer, upper and lower ground layers, and two dielectric layers. The signal layer includes a differential pair comprising two transmission lines to transmit a pair of differential signals. The dielectric layers are located on and under the signal layer to sandwich the signal layer. The upper ground layer is attached to the dielectric layer on the signal layer, opposite to the signal layer. The lower ground layer is attached to the dielectric layer under the signal layer, opposite to the signal layer. Each ground layer includes a grounded sheet made of conductive material. Two voids are defined in each ground layer and located at opposite sides of the corresponding grounded sheet. Distances between the middle line of the grounded sheet of each ground layer and middle lines of the two transmission lines are equal.