This paper discusses two issues including a new power-combined technique and efficiency enhancement technique in mm-wave power amplifier design. A new concept of "folded-transformer" is proposed to reduce effectively the active area of power combiner and splitter. Furthermore, the efficiency enhancement technique at power-back operations is presented to extend the battery service time of the mobile device. The 60-GHz PA adopts an ultra-compact 4-way folded-transformer based architecture and provides power level-dependent multi-operating modes. To the best of the author's knowledge, this is the first mm-wave power amplifier with efficiency enhancement technique at power back-off operations. The PA is implemented in TSMC 90nm low-power CMOS process. While 1.2 V supply, the measurement results show that the peak power is 18.5 dBm at 60GHz and the drain efficiency of the power stage achieves of 24.1%. At 6dB/10dB power back-off operation, the drain efficiencies of power stage can be enhanced from 5.9% / 2.4% to 11.9% / 8%, respectively.