A novel push-push oscillator taking advantages of "Double-Sided MIC Technology" is proposed. The oscillator incorporates microstrip lines on a dielectric substrate and a slot line on the reverse side. By integrating a slot line and microstrip lines, the push-push oscillator can be realized very easily. All the concerned undesired harmonic signals (f0, 3f0 and 4f0) can be suppressed satisfactorily. Using these approaches, a push-push oscillator in K-band is designed and fabricated. The output power of +4.17 dBm at the frequency of 21.25 GHz is measured. All the undesired signals are sufficiently suppressed to be better than -30 dBc. The phase noise is -99.68 dBc/Hz at the offset frequency of 1 MHz.