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 was implemented by a standard 90-nm RF CMOS technology. The measured flat gain is more than 10dB from 28.5 to 39GHz. The -3dB bandwidth is 15GHz from 27 to 42GHz which covers almost the entire Ka band. The minimum noise figure (NF) is 4.2dB, and the average NF is 5.1dB within the 27～42GHz range. The S11 is better than -11dB over the overall testing band. The input 3rd-order intermodulation point (IIP3) is +2dBm at 40GHz. The DC power dissipation of the whole circuit is as low as 5.3mW. The chip occupies an area of 0.58mm×0.48mm including all pads.