In this letter, we presented a frequency synthesizer with ultra-low phase noise in 2-µm GaAs HBT process. To obtain lower phase noise and better start-up condition, a gm-boosted Colpitts VCO is proposed to mitigate the out-band phase noise. To validate the proposed idea, a prototype operating at 1.6 GHz is designed, and fabricated. The measured results show that a phase noise of -104.35 dBc/Hz@10kHz and -130.8 dBc/Hz@1MHz with reference spur of -73 dBc are achieved, demonstrating great competitiveness over than other tradition ones. The proposed frequency synthesizer is a potentially ideal candidate for low phase noise and radiation-hardened applications.