This paper presents CMOS active transformers synthesized using MOS transistors only and their applications in VCOs. The characteristics of CMOS active transformers are analyzed, and the self and mutual components of the quality factor of transformers are investigated. Simulation results of LC, quadrature, and transformer VCOs with active inductors and active transformers show that active transformer VCOs exhibit a much lower level of phase noise as compared with corresponding LC and quadrature VCOs with active inductors. The phase noise of the designed 1.5GHz active transformer VCO in TSMC-0.18μm 1.8V CMOS technology is -109.4dBc/Hz at a 1 MHz offset the carrier and is comparable to that of VCO with spiral inductors. The power consumption of the oscillator is 30.5 mW with 83 μm2 active area.