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This paper designs and implements an direct up-conversion mixer based on RF CMOS process, which can be used in the up-link radio frequency (RF) front-end of a 2.4 GHz wideband wireless system such as multimedia wireless sensor network (WSN). Firstly, Gilbert cell-based double-balanced structure is adopted in this design, which can effectively suppress the feed-through of the local oscillator (LO) signal to the RF port, and as well as that of the intermediate frequency (IF) signal to the RF port. Then, an improved Gilbert cell structure-based circuit is proposed by introducing source degeneration technique to improve the linearity of the up-mixer. LC resonant network is used as load of the mixer to select the first-order mixing items. Finally, the mixer is fabricated in TSMC 0.18μm 1P6M RF CMOS process. Measured results show that the mixer performs well and has better linearity, the input 1dB compression point is -1dBm, and the conversion gain is -0.5 dB with the total current (including the mixer core and the buffer) of 16 mA. The entire chip area (including the pads) is 780μm × 560μm.