This article presents an ultralow power K-band frequency doubler operating under 0.4-V supply voltage. With 0-dBm input signal, the frequency doubler consumes 2.4 mW and achieves −8.4-dB conversion gain (CG). The measured saturated output power is −6 dBm at 23.6 GHz, and the 3-dB bandwidth is from 20.4 to 25.4 GHz. It exhibits up to 39-dB fundamental rejection (FR). The proposed differential frequency doubler operates at an ultralow supply voltage with ultralow power consumption in active frequency doublers.