This article presents a voltage-controlled oscillator (VCO) with wideband-differential-mode (DM) second harmonic resonance. It features a single-turn multitap inductor plus a small tail resonator to boost the drain-to-gate voltage gain (A_V). It also creates intrinsic-high-Q impedance peaks at the fundamental and double oscillation frequencies, which simultaneously achieves phase noise (PN) and 1/ f³ PN-corner reduction. Prototyped in 65-nm CMOS, the 5.99-GHz VCO scores a −138.9 dBc/Hz PN at an offset of 10 MHz and consumes 3.17 mW of power with a 0.67 V supply. The achieved figure-of-merit (FoM) values were 177.5, 186.6, and 189.4 dBc/Hz at an offset of 0.1, 1, and 10 MHz, with a 1/ f³ PN corner of 400 kHz. Over a 14% tuning range (TR), without any additional harmonic tuning, the VCO upholds a consistent FoM > 189 dBc/Hz at an offset of 10 MHz. The core area is 0.465 mm².