This paper presents a direct-conversion I/Q up-mixer block, which supports $3{sim}5$ GHz ultra-wideband(UWB) applications. It consists of a VI converter, a double-balanced mixer, a RF amplifier, and a differential-to-single signal converter. To achieve wideband characteristics over $3{sim}5$ GHz frequency range, the double-balanced mixer adopts a shunt-peaking load. The proposed RF amplifier can suppress unwanted common-mode input signals with high linearity. The proposed direct-conversion I/Q up-mixer block is implemented using $0.18-{mu}m$ CMOS technology. The measured results for three channels show a power gain of $-2{sim}-9$ dB with a gain flatness of 1dB, a maximum output power level of $-7{sim}-14.5$ dBm, and a output return loss of more than - 8.8 dB. The current consumption of the fabricated chip is 25.2 mA from a 1.8 V power supply.