In this brief, a transformer-type power combiner for a fully integrated high-power CMOS power amplifier (PA) is presented. The proposed power combiner is composed of a number of transformers that, unlike the ones in conventional approaches, have different sizes. This leads to higher efficiency and smaller chip area. After considering several power stage topologies, analysis and optimization of the transformer network (the power combiner) are presented. To demonstrate the advantages of the proposed architecture, a 900-MHz CMOS PA with the proposed power combiner was implemented with a 0.18-μm radio-frequency CMOS process. The amplifier achieved an efficiency value of 24% at the maximum output power of 29.5 dBm.