A novel tri-band up-mixer was designed for EoC transceiver chips based on 0.13 μm RF CMOS process. In the design, the mixer was working at 1.2 GHz, 2 GHz and 2.4 GHz bands under different parallel LC resonant network by changing the output loading capacitance. Under 3.3 V power supply, the up-mixer had a power consumption of 35.1 mA, the SSB voltage conversion gain was 3.77 dB, 4.97 dB and 4.78 dB at each band of 1.2 GHz, 2 GHz and 2.4 GHz, the OP1dB was -0.22 dBm, 0.78 dBm and 0.5 dBm, and the noise figure was 5.13 dB, 5.76 dB and 6.67 dB respectively. The up-mixer could turn on and off by controlling the bias of the input transconductance stage, the total turn-on time was about 200 ns and the turn-off time was less than 100 ns.