A triple-mode Fractional-N frequency synthesizer with a noise-filter voltage controlled oscillator (VCO) for WCDMA/Bluetooth/ZigBee applications has been implemented in 0.18-μm RF-CMOS technology. The proposed synthesizer achieves a good phase noise lower than -80dBc/Hz in band and -115dBc/Hz@1MHz for the three modes, and only draws 21mA from a 1.8V supply. It has a high hardware sharing and a small size, only 1.5mm×1.4mm. The system architecture, circuit design and measured results are presented in this paper.