This paper reports a high power 60 GHz push-push oscillator fabricated using $0.12{mu}m$ metamorphic high electron-mobility transistors(mHEMTs). The devices with a $0.12{mu}m$ gate-length exhibited good DC and RF characteristics such as a maximum drain current of 700 mA/mm, a peak gm of 660 mS/mm, an $f_T$ of 170 GHz, and an $f_{MAX}$ of more than 300 GHz. By combining two sub-oscillators having $6{ imes}50{mu}m$ periphery mHEMT, the push-push oscillator achieved a 6.3 dBm of output power at 59.5 GHz with more than - 35 dBc fundamental suppression. The phase noise of - 81.5 dBc/Hz at 1 MHz offset was measured. This is one of the highest output power obtained using mHEMT technology without buffer amplifier, and demonstrates the potential of mHEMT technology for cost effective millimeter-wave commercial applications.