This letter presents a D-band low-power gain-boosted up-conversion mixer using the 40-nm complementary metal-oxide-semiconductor technology. The proposed up-conversion mixer adopted gain-boost technique to improve conversion gain with a low local oscillator (LO) power. The resistive-feedback inverter was employed for wideband matching at intermediate-frequency ports. Broadband Marchand baluns were used to transform a single-ended signal to a differential signal at the RF and LO ports for measurement. The proposed up-conversion mixer demonstrated a measured conversion gain of -5 dB ± 1 dB at frequencies from 105 to 135 GHz with an ultralow LO power of -10 dBm. The average LO-RF isolation was -35 dB. The measured input P1dB compression point was -7.5 dBm. This core chip occupies 560 x 400 μm² and the total power consumption is 9 mW from a 1-V supply voltage.