Abstract: A flexible printed circuit comprises a flexible insulative board having a first surface and a second surface opposite thereto. At least one conductive transmission line is formed on the first surface and comprises same length of a plurality of tested sections. Each tested section has an original characteristic impedance and the original characteristic impedance of each tested sections is beyond an acceptable tolerance to a standard value. A compensation region is defined at the second surface and separated into a plurality of compensation units having identical area. Each compensation unit is vertically related to a specific tested section of the at least one transmission line and having a conductive region and a non-conductive region. A final characteristic impedance of the tested section is obtained within an acceptable tolerance to the standard value due to compensation of the compensation unit.