This paper presents the application of switchable inductive tuning technique to the frequency tuning range (FTR) enhancement for millimeter-wave oscillators. By rearranging the induced-current path in the guard ring of a planar inductor, the inductance value can be changed and therefore the resulted total tuning range of the oscillator can be extended. Two VCOs are fabricated for demonstrating the effectiveness of this tuning technique. By using 90nm CMOS technology, a V-band 55 GHz direct VCO with an FTR of 9.95 GHz and a W-band push-push 109 GHz VCO with an FTR of 17.8 GHz are achieved. For the V-band VCO, the measured phase noise ranges from -110 to -120 dBc/Hz over the whole tuning range, no matter the switches are either turned on or off. The power consumption is 11.3 mW with a 0.9 V supply. The evaluated FOM is -184.2 dBc/Hz and FOMT is -189.3 dBc/Hz. For the W-band VCO, the measured phase noise is -108.9 dBc/Hz at 10 MHz offset from the carrier frequency of 100.6 GHz, while it consumes 22.8 mW from a 1.2 V supply. The evaluated FOM is -175.4 dBc/Hz and the FOMT is -179.5 dBc/Hz.