In this work we report a re-configurable differential voltage-controlled oscillator (VCO) suitable for the GSM (1.8 GHz) / WLAN (2.4 GHz) co-existence system application. The oscillator, containing a switchable inductor as well as a two-bit switched capacitance array, is fabricated in a 0.18 μm RF CMOS technology. The measurement results exhibit the tuning ranges from 1,487 MHz to 1,866 MHz as the inductor switch is OFF and from 1,952 MHz to 2,510 MHz as the switch is ON, with the tuning voltage from 0.3 V to 1.8 V. In the GSM band, the measured phase noises are about -119 dBc/Hz at 600 KHz offset and -133 dBc/Hz at 3 MHz offset. In the WLAN band, the corresponding phase noises at 100 KHz and 1 MHz offsets are superior to -93 dBc/Hz and -119 dBc/Hz, respectively. The DC power dissipation of the oscillator core is about 44.6 mW at the supply voltage of 2.5 V. The figure of merits (FOM) of this VCO is about-172 dB.