A frequency synthesizer capable of using a voltage controlled oscillator (VCO) with a low value of gain Kvco, having a short automatic frequency calibration time and automatically coping with environments and a frequency calibration method are provided. The frequency synthesizer includes a reference divider, a phase detector, a main divider, a charge pump, a loop filter, a first switch, a second switch, a VCO, and an automatic frequency calibration block. The frequency calibration method includes an initialization step, an N-target algorithm step, an auxiliary search algorithm step, and a post search algorithm step.