A novel defected ground structure(DGS) was designed and applied to a flip-chip integrated millimeter wave oscillator.The characteristics of two oscillators with and without DGS were analyzed and compared.Measurement data shows that the phase noise of the oscillator with DGS is reduced by 4~6dB,and the output power of the oscillator is increased by 0.8dBm in comparison with the oscillator without DGS.It is found that,when DGS is embedded in the resonant tank and the output terminal of an oscillator,the phase noise can be reduced and the output power will be enhanced.