A K-band low power voltage-controlled oscillator (VCO) using transformer coupled dual LC tanks is implemented in a 90-nm CMOS process. The use of transformer provides tight coupling factor between two LC tanks that improves the phase noise performance by increasing the output signal swing and waveform symmetry of the VCO. In addition, taking advantage of the tight-coupling transformer, the inductor layout is properly designed to obtain a high Q-factor and a die area comparable to single-inductor VCO. The VCO core dissipates very small DC power of 1.61mW from a 0.7-V supply due to the benefit of tight-coupled transformer. The measured phase noise is -97.61 dBc/Hz at 1-MHz offset from 23.99 GHz oscillation frequency.