This paper presents a novel, dual-mode, reconfigurable, fully integrated double-balanced ring mixer. The change in DC bias of the mixer core leads to two operation modes: a passive mixer and an active mixer with variable conversion gain. In addition, an active IF balun using a DC-coupled differential pair configuration is designed to cover IF frequency from DC to 4 GHz. The measurement results show that the conversion gain can change from −6 dB to 6 dB over RF frequency range of 742 GHz. The measured input third order intercept point (HP3) is 5 dBm and the measured input 1-dB compression point (IP1dB) is 5 dBm at the highest conversion gain mode with 4-dBm LO drive. At the lowest conversion gain mode, the HP3 is 20 dBm and IP1dB is 12 dBm with 13-dBm LO drive.