This article proposes a quad-core millimeter-wave (mm-wave) voltage-controlled oscillator (VCO) using a compact multi-mode magnetically coupled LC network. The network can be configured to achieve four different resonant modes. In conjunction with the mode switches, four VCO cores are coupled with each other to achieve the quad-mode switching; thus, the tuning range (TR) is widened. In addition, the mismatch and phase noise (PN) of the asymmetrical multi-core oscillator have been analyzed. Implemented in 65-nm CMOS process, the VCO achieves a measured continuous TR of 90.2%, from 12.1 to 32.0 GHz. The measured best PN is −103.2 dBc/Hz at 13.2 GHz with 1-MHz offset, corresponding to a figure-of-merit (FoM) of −177.6 dBc/Hz and an FOM of TR and area (FoM_TA) of −207.7 dBc/Hz. This chip operates at a 0.8-V supply with a power consumption of 5.6–22.5 mW and a core area of 0.08 mm².