A two-stage 0.18μm CMOS power amplifier (PA) for ultra-wideband (UWB) 3 to 5 GHz using common source inductive degeneration is presented in this paper. On-wafer measurement shows an average power gain of 15.2 dB with gain flatness of 0.6 dB and an input 1 dB compression (P_1dB) above - 6.1 dBm from 3 to 5 GHz while consuming 25 mW from a 1.8 V supply. Load-pull measurement also shows a power added efficiency (PAE) of 34% at 4 GHz with 50Ω load impedance. Results obtained in this work could be used as a reference design for immediate PA implementation in commercial mobile or portable UWB transmitter or signal generator.