A millimeter-wave down-converter employing an on-chip balun transformer with improved common-mode rejection ratio is presented for 5G direct-conversion receiver. The down-converter consists of a down-conversion mixer and local-oscillator (LO) buffer. The down-converter was implemented in a 65-nm CMOS process. It consumes a power of 13.7 mW from a 1-V supply voltage. It shows a gain greater than 11 dB, a noise figure of approximately 9 dB and a third-order output intercept point higher than 13 dBm when LO input power is -10 dBm in the frequency ranges from 26.5 GHz to 29.5 GHz.