A class D amplifier and a method of pulse width modulation are provided. The class D amplifier includes a sigma delta modulator, a reference signal generator, a pulse width modulator and an amplifier. The sigma delta modulator converts an M bit input signal into N bit pulse code modulation data (M and N are integers and M>N). The reference signal generator counts a system clock and generates an N bit reference signal having 2N data blocks in a sampling period of the pulse code modulation data. The pulse width modulator generates a plurality of pulse width modulation signals having at least two switching periods within the sampling period of the pulse code modulation data. The pulse width modulation signals have pulse widths varied based on a unit time interval of the reference signal. The unit time interval corresponds to one of the data blocks of the reference signal.