In this paper, a novel Gunn diode push-push oscillator is proposed. Gunn Diodes and a slot ring resonator are used for the push-push oscillator. The combination of the Gunn diodes and the resonator is considered to be an active resonator. A double-sided circuit technology is adopted in this oscillator. Two Gunn Diodes are arranged on the slot line of the ring resonator on a surface of a substrate. The microstrip line formed on the reverse side is coupled with the slot ring resonator magnetically. The oscillator is designed and fabricated in K-band. As for the experimental results, the output power of −0.9 dBm at the second harmonic frequency of 21.6 GHz is obtained. The measured phase noise is −108.8 dBc/Hz at 1 MHz offset frequency. The oscillator is designed easily due to the very simple structure and achieves low phase noise performance with low cost by using the advantages of Gunn diode oscillators.