A 28 GHz sub-harmonically pumped passive down conversion mixer fabricated in a 0.18-μm CMOS process is demonstrated. A low power fully differential LO frequency doubler is designed to generate at near half RF frequency. The proposed sub-harmonically pumped passive mixer has advantages in low power consumption, high fundamental frequency suppression, and suitable apply to millimeter-wave frequencies. At 28 GHz RF frequency and 13.2 GHz LO frequency, the measured conversion loss of the mixer is less than 11 dB, single side band noise figure of 11.6 dB, the isolations among LO, IF and RF are over 33 dB, and a third-order intercept point at the input of 8 dBm, while dissipating total current of 0.6 mA from 1 V supply. To the authors' knowledge, the design achieves the highest figure of merit among published down-conversion mixers operating at similar millimeter-wave frequencies in comparable silicon based technology.