A W-band double-balanced mixer for direct up-conversion using standard 90 nm CMOS technology is reported. The mixer comprises an enhanced double-balanced Gilbert cell with PMOS negative resistance compensation for conversion gain (CG) enhancement and current injection for power consumption reduction and linearity improvement, a Marchand balun for converting the single LO input signal to differential signal, another Marchand balun for converting the differential RF output signal to single signal, and an output buffer amplifier for loading effect suppression, power consumption reduction and CG enhancement. The mixer consumes low power of 7.3 mW and achieves IF-port input reflection coefficient smaller than -8.8 dB for frequencies lower than 3 GHz, and LO-port input reflection coefficient of -8.8~ -23.9 dB and RF-port input reflection coefficient of -11.2~ -22.4 dB for frequencies of 70~90 GHz. For RF frequencies of 77~81 GHz, the mixer achieves CG of 6.25±1.05 dB and LO-RF isolation of 63.9±1.6 dB, the best CG and LO-RF isolation results ever reported for a CMOS/BiCMOS up-conversion mixer with operation frequency around 80 GHz.