An innovative on-chip 3-D power amplifier (PA) architecture for M-way power-combined CMOS PAs by using the proposed dual-radial symmetric architecture is presented. It provides design freedom of impedance selection of power device in transformer-based millimeter-wave PA design. This idea also makes distinguished breakthrough to the traditional 2-D PA architecture without compromising symmetry and compact size of layout. To demonstrate the feasibility of this idea, a 60-GHz PA is fabricated in 90-nm low-power CMOS process. It is also equipped with multi-mode operation. It achieves an output power of 18.5 dBm and a power-added efficiency of 10.2% with 1.2-V supply voltage. At 6-dB/10-dB power back-off operation, the drain efficiencies of power stage can be enhanced from 5.9%/2.4% to 11.9%/8%, respectively, by enabling the multi-mode operation.