In this paper, a fully integrated 5GHz voltage controlled oscillator (VCO) is presented. The VCO is designed with 0.18µm silicon complementary metal oxide semiconductor (Si-CMOS) process. To achieve low phase noise, a novel varactors pair circuit is proposed to cancel effects of capacitance fluctuation that makes harmonic currents which increase phase noise of VCO. The VCO with the proposed varactor circuit has tuning range from 5.1GHz to 6.1GHz (relative value of 17.9%) and phase noise of lower than −110.8dBc/Hz at 1MHz offset over the full tuning range. Figure-of-merit-with-tuning-range (FOMT) of the proposed VCO is −182dBc/Hz.