This paper shows the innovational design of a differential transmission-line inductor (DTL-inductor) for the millimeter-wave voltage-controlled oscillators. A non-uniform type of DTL-inductor (Non-UDTL-inductor) with slow-wave floating metal strips is proposed for the compact-size design of the LC-resonator while having a high quality factor. Besides, a novel concept to electrically switch the equivalent inductance of the DTL-inductor is also proposed. Using a 0.13-μm RF CMOS process, a V-band 56 GHz VCO is demonstrated with the integration of such a Non-UDTL-inductor. The measured results exhibit a tuning range from 57.09 to 55.06 GHz and 56.91 to 54.9 GHz in the "switch-ON" and "switch-OFF" states, respectively, as the tuning voltage rising from 0 V to 1.2 V. The phase noise level is around -95 dBc/Hz at 1-MHz offset and the output power level ranges from -13.6 dBm to -10 dBm over the whole tuning range. The DC power consumption of the oscillator core is 9.9 mW from a 1.2 V supply voltage. The calculated figure of merits (FOM) is about -180 dB.