This paper presents a 76～81 GHz CMOS millimeter wave quadrature down-mixer for automotive radars. The transformer-based gm-boosted method is utilized in the pre-amplifier for improving gain performance. A parallel inductor and two cross-coupled transistors with dynamic current-bleeding are combinatorially exploited to resonate the parasitic capacitance between the transconductance stage and the switching stage and improve noise figure and conversion gain. The proposed mmW down-mixer is implemented in a 55-nm CMOS process and the measurement results show that the BW_-3dB is 5.5 GHz, the peak gain is up to 4.1 dB with <0.16 dB I/Q mismatch （at 50 Ω load）, the input P1dB is -6 dBm and the minimum noise figure is of 19 dB while consuming 40 mW of power. Excellent FOM is achieved by combining above-mentioned techniques.