A circuit topology suitable for low-power Colpitts voltage-controlled oscillators (VCOs) is presented in this letter. By employing the proposed voltage-to-current positive-feedback network, the required transconductance for VCO startup can be reduced, leading to the minimized dc power for sustaining VCO oscillation. Moreover, the Q-factor enhanced varactor is used in this VCO design for phase noise improvement. Based on the proposed architecture, the fabricated VCO in standard 0.18 μm CMOS exhibits a 3.58% tuning range. Operating at 1.35-V supply voltage, the VCO core consumes 3.3-mW dc power. The measured phase noise is -110.82 dBc/Hz at 1 MHz offset from 18.95 GHz oscillation frequency. Compared with the recently published K-band 0.18 μm CMOS VCOs, it is observed that the proposed Colpitts VCO exhibits comparable circuit performance under low dc-power consumption.