A controller of a power amplifier includes an oversampler for oversampling an incoming digital signal to generate an oversampled signal, a controlling module for generating a first control signal and a second control signal according to the oversampled signal, a PWM circuit for generating a first PWM signal according to the first control signal, and a pulse width adjuster for adjusting a pulse width of the first PWM signal according to the second control signal to generate a second PWM signal for controlling a power stage of the power amplifier.