This letter presents a high efficiency, and small group delay variations 12-24 GHz fully-integrated CMOS power amplifier (PA) for quasi-millimeter wave applications. Maximizing the power added efficiency (PAE), and minimizing the group delay variations in a wideband frequency range are achieved by optimizing the on-chip input, output, and inter-stage matching circuits. In addition, stagger tuning is employed for realizing excellent gain flatness. A two-stage CMOS PA using the proposed methodology is designed and fabricated in 0.18 µm CMOS technology and tested. A measured power gain (|S 21 |) of 10.5 ² 0.7 dB and a measured small group delay variation of ²20 ps over the frequency range of interest are achieved. The PA shows a maximum measured PAE to be 26 % with DC power consumption of 50 mW.