A low noise-figure (NF) and high power gain (S21) 3 ~ 10 GHz ultra-wideband low-noise amplifier (UWB LNA) with excellent phase linearity using 0.18 μm CMOS technology is reported. An enhanced π-match input network is used to achieve wideband input impedance matching as well as high and flat S21. To achieve low and flat NF, the pole frequency and pole quality factor of the second-order NF frequency response are tuned to approximate the maximally flat condition. The LNA consumes 18 mW, achieving S11 better than -10 dB for frequency lower than 12.2 GHz and group-delay (GD) variation smaller than ±14.6 ps for frequencies 3 ~ 10 GHz. Additionally, high and flat S21 of 13.7 ± 1.5 dB is achieved for frequencies 1 ~ 12.5 GHz, which means the corresponding 3-dB bandwidth is 11.5 GHz. Furthermore, the LNA achieves minimum NF of 2.2 dB at 4 GHz and NF of 2.3 ± 0.1 dB for frequencies of 3 ~ 10 GHz, one of the best NF results ever reported for a 3 ~ 10 GHz CMOS LNA.