In this paper, a novel GHz wideband common-mode suppression circuit based on transmission line theory has been proposed. The odd even-mode equivalent circuits have been built by using two-wire transmission line model, which predict the circuit performance well in a wide frequency range. According to experiment results, the common-mode noise can be suppressed over 10 dB from 1.9 to 4.5 GHz with 81% fractional bandwidth while the insertion loss of differential-mode can be kept less 3dB from dc to 6 GHz. Also, the group delay for the differential-mode is constant within the broad frequency range. It is important that the proposed common-mode suppression circuit can keep good signal integrity of the digital differential signals due to its all-pass characteristic.