This paper presents an Ultra-wideband (UWB) low-noise amplifier (LNA) based on a single-ended common-gate (CG) in cascade with cascode configuration. The proposed LNA is implemented by a standard 90-nm RF CMOS technology and it features that the measured flat gain is more than 10 dB from 28.5 to 39 GHz, the -3 dB bandwidth is 15 GHz from 27 to 42 GHz which covers almost the entire Ka band, the minimum noise figure (NF) is 4.2 dB, the average NF is 5.1 dB within the 27-42 GHz range, the S₁₁ is better than -11 dB over the overall testing band, and the input 3rd-order intermodulation point (IIP3) is 2 dBm at 40 GHz. The DC power dissipation of the whole circuit is as low as 5.3 mW. The chip occupies an area of 0.58*0.48 mm2 _{including all pads.}