In this paper, we propose a quad-core triple-mode VCO with switched inductors. Two identical single-turn coils with high Q factors are implemented symmetrically and are magnetically coupled with each other. Three equivalent inductances are achieved by controlling the current directions in two coils through mode switches. An automatic-mode-tracking buffer with two magnetically coupled inductors is proposed to boost the output amplitude of the VCO and save power. Two stages of cascaded dividers are used for covering a wider output frequency range. Fabricated in a 65-nm CMOS process, the proposed VCO achieves a TR of 65.5% from 18.5 GHz to 36.5 GHz. Under a supply voltage of 0.45 V, the VCO core consumes a power of 3.9 to 5.2 mW. The measured phase noise at 10 MHz offset is -121.6 to -130.3 dBc/Hz. The measured peak FoM is 190.5 dBc/Hz at 10 MHz offset and peak FOM T is 206.8 dBc/Hz.