A 6-10-GHz broadband CMOS power amplifier (PA) for Ultra-wideband (UWB) transmitters is presented. An inter-stage wideband impedance transformer is designed between the small-signal stage and driver amplifier for the wideband operation. The UWB PA is implemented in the 0.18-μm 1P6M standard UMC CMOS process. Fully on-wafer measurement shows that the fabricated PA exhibits an average small-signal gain, P1dB, and PAE of 8.5 dB, 5 dBm, and 14.4%, respectively, from 6 to 10 GHz. A low power consumption of only 18 mW is obtained from a voltage of 1.5 V.