A frequency shift keying modulator having sigma-delta modulated phase rotator is disclosed, which includes a phase-locked loop for generating a voltage-controlled signal; a multi-phase generator for receiving the voltage-controlled signal and generating N phase-shift signals having same frequency according to the voltage-controlled signal, the N phase-shift signals having a same phase shift between the phase-shift signals adjacent to each other; a sigma-delta modulator for receiving transmission data and randomly outputting a modulation bit at a modulation clock according to the transmission data; and a phase rotator for receiving the N phase-shift signals and selectively outputting one of the N phase-shift signals and a frequency-divided signal according to the modulation bit, wherein the frequency of the frequency-divided signal is 1 / ( 1 + n N ) of the frequency of any one of the N phase-shift signals.