In this letter, we design and analyze 0.1–1.5 GHz multi-octave quadruple-stacked CMOS power amplifier (PA) in 0.18 μm CMOS technology. By using two-stage quadruple-stacked topology and feedback technology, the proposed PA realizes an ultra-wideband CMOS PA in a small chip area. Wideband impedance matching is achieved with smaller chip dimension. The effects of feedback resistors on the RF performance are also discussed for this stacked-FET PA. The PA shows measured input return loss (< –10.8 dB) and output return loss (< –9.6 dB) in the entire bandwidth. A saturated output power of 22 dBm with maximum 20% power added efficiency (PAE) is also measured with the drain voltage at 5 V. The chip size is 0.44 mm² including all pads.