A novel dynamic-bulk-bias (DBB) varactor is proposed, to increase frequency tuning rang of a VCO. Different from the conventional inversion-mode varactor, the DBB structure can stabilize the voltage across the varactor, which ensures the varactor to operate in the strong inversion region and gains the largest capacitance tuning range. The measurement result shows the proposed design increases the frequency tuning range by 2 times larger than that of the conventional VCOs. Here, a 5 GHz VCO is designed and fabricated by using a standard 0.18 um CMOS process. From a 0.6-V supply voltage, the VCO exhibits a frequency tuning range of 20%, a phase noise of -118 dBc/Hz at 1-MHz offset frequency and a power consumption is 4 mW. The result also shows that the DBB based VCO can operate at a lower supply voltage.