A novel variable inductor using mutual current control was developed and applied to a 4.5-GHz SiGe voltage controlled oscillator (VCO). It consists of a primary inductor, a secondary inductor, and a variable capacitor, and its inductance can be tuned by adjusting the capacitance of the capacitor. It was designed for application to a proposed 4.5-GHz VCO in 0.25-μm SiGe BiCMOS technology. It exhibited a large inductance variation of 22% and a high quality factor of 24 at 4.5 GHz in electro-magnetic simulation. The VCO has a 1.2-GHz tuning range, 32% wider than that of a conventional VCO, and a low phase noise of -121.8 dBc/Hz (at a 1-MHz offset frequency).