A CMOS broadband millimeter-wave power amplifier (PA) based on a Sandwiched Transformer (ST) output matching network is presented in this paper. The ST output matching network with a three-layer structure providing a larger coupling coefficient (k) than the traditional two-layer stack structure is proposed for PA's output matching network. The layout of the transistors is optimized to improve the PA's performance. Fabricated in 65-nm CMOS process, the PA has achieved 15 dBm OP 1dB and 36.5% peak power added efficiency (PAE). The 3-dB bandwidth of the PA is from 22.8 GHz to 32.8 GHz.