This paper presents the design and analysis of a 21~29 GHz CMOS receiver front-end (RFE) in a standard 0.18 μm CMOS process for ultra-wideband (UWB) automotive radar systems. The circuit comprises a low-noise amplifier (LNA), a double-balanced Gilbert-cell mixer, and two Marchand baluns. The performance of the mixer was improved with the current-bleeding technique and a parallel resonant inductor in the input trans-conductance stage. The wideband Marchand baluns can not only convert the single RF and LO signals to nearly perfect differential signals over the 21~29 GHz band but also enhance the port-to-port isolations. Over the 21~29 GHz automotive radar band, the RFE exhibited excellent NF of 4.6±0.5 dB, conversion gain of 23.7±1.4 dB, RF port reflection coefficient of -8.8~-16.8 dB, LO-IF isolation of -47~-52.3 dB, LO-RF isolation of -55~-70.5 dB, and RF-IF isolation of -35.5~-45.4 dB. The circuit occupied a chip area of 1.15×0.93 mm², i.e. 1.08 mm², excluding the test pads. The dc power dissipation was only 39.2 mW.