In this paper, we present a novel gain-enhanced sub-harmonic mixer based on 0.5-μm emitter width InGaAs/InP double heterojunction bipolar transistors (InP DHBTs). The proposed mixer consists of a transconductance stage and a gain-enhanced stage. A common-emitter transistor is used in the first stage to realize the sub-harmonic mixing while another common-emitter transistor is used in the second stage to remix the f _LO+IF and fIF and also amplify the f _2LO+IF . For further verification, a transconductance mixer and a gain-enhanced mixer were designed and fabricated. Compared with the transconductance mixer, the gain-enhanced mixer exhibits a 6.8-dB higher conversion gain with 2-dB lower LO input power and a peak up-conversion gain of 9 dB at 213 GHz with f _IF = 1 GHz, f _LO = 106 GHz, and P _IF = -26 dBm P _LO = 3 dBm. To our best knowledge, the gain-enhanced mixing structure is proposed for the first time.