A broadband microwave/millimeter-wave (MMW) Gilbert-cell mixer using standard 1P8M 0.13-μm complementary metal oxide semiconductor (CMOS) technology is presented in this letter. Two radio frequency (RF) transformer baluns are used in RF- and local oscillator (LO)-ports to convert single-ended signals to differential signals. Thin film microstrip line is employed for the matching networks and transformer design. This mixer has a conversion gain of better than 5 dB from 9 to 50 GHz. Between 5 and 50 GHz, the RF- and LO-to-intermediate frequency (IF) isolations are better than 40 dB. The RF-to-LO and LO-to-RF isolations are all better than 20 dB. To the authors' knowledge, this is the first CMOS Gilbert-cell mixer operating to MMW frequency to date.