This paper presents the implementation of a transformer-based 3-5-GHz UWB mixer in 0.18-μm CMOS process. An on-chip transformer being inserted between the transconductance and the switch stages, the proposed mixer satisfies the demands of low supply voltage and low power consumption in UWB applications. The RF transconductance stage of the mixer includes a capacitive cross-coupling to widen the working frequency to be 3 - 5 GHz. The measured results show that the mixer has a maximum conversion gain of 4.4 dB with a power consumption of 16 mW under a supply voltage of 0.8 V. The measured LO-IF, LO-RF and RF -IF isolations are 36 dB, 44 dB and 40 dB, respectively. The chip size is 1.78 × 1.39 mm2.