In this paper, TSMC 0.25(m CMOS technology is used to implement a low-phase-noise quadrature VCO, which is suitable for an 802.11a wireless local area network (WLAN). The four phases of VCO outputs are 90(, 180(, 270( and 360(, respectively. As the simulation result shows, the tuning frequency is 4.441GHz~5.323GHz, the phase noise of the design is -122.741dBc/Hz offset 1 MHz, and the tuning range is 0.882GHz, which is about 18%. The output power is -7.67dBm, and the power consumption is 15.8mw.