In this paper, we propose a K-band voltage controlled oscillator (VCO) using defected ground structure (DGS) that realizes the series resonant and parallel resonant characteristic. An H-shaped DGS etched at the lowest metal layer (M1) below microstrip line (MSL) on the top metal layer (M6) of 0.18 μm 1P6M CMOS process. The MSL is etched in the middle to realize series capacitance, which produces series resonance characteristics and sharpens the response of magnitude of Z11 (mag|Z11|). The proposed DGS resonator achieved smaller size and high quality (Q-) factor. The post-layout simulation result shows that VCO has a tuning range of 3.4%, and a low phase noise with -110.9 dBc/Hz @1 MHz offset at 21.02 GHz oscillation. The VCO consumes 7.5 mW power resulting in a FoM of 188.7 dB.