This letter presents a 48 GHz frequency doubler in a 65 nm CMOS technology. The proposed frequency doubler is composed of a complementary push-push structure with negative resistance circuit for conversion gain enhancement. The maximum measured conversion gain reaches −6.1 dB at 48 GHz output frequency, and the 3-dB bandwidth is 40∼54 GHz. The fundamental rejection is above 29.5 dB. The size of the proposed frequency doubler chip is 0.72 × 0.36 mm2The total power consumption is 16 mW.