The invention provides a light-emitting diode (LED) driver and a driving method thereof. The LED driver includes a brightness code calculation circuit, a driving current generation circuit and a control circuit. The brightness code calculation circuit is configured to to receive a pulse width modulation (PWM) signal and perform a calculation operation comprising calculating a duty cycle of the PWM signal with respect to a period of the PWM signal and generating a brightness code according to the duty cycle of the PWM signal. The driving current generation circuit is configured to generate a driving current according to the brightness code. The control circuit is configured to detect a plurality of brightness codes generated by the brightness code calculation circuit repeatedly performing the calculation operation and determine whether or not to adjust an execution frequency of the calculation operation of the brightness code calculation circuit according to the brightness codes.