A DC-DC converter used to convert an input voltage to an output voltage is disclosed. The DC-DC converter comprises a pulse-width-modulation (PWM) generator, a transient boost circuit, a logic circuit, a switching device, and a buck circuit. The pulse-width-modulation (PWM) generator generates a PWM signal according to the output voltage. The transient boost circuit generates an adjusting signal according to the variation of the output voltage. The logic circuit generates a switch signal according to the PWM signal and the adjusting signal. The switching signal is at a high level when the PWM signal or the adjusting signal is at the high level, and the switching signal is at a low level when the PWM signal and the adjusting signal are at the low level. The switching device converts the input voltage to a driving signal according to the switching signal. The buck circuit receives the driving signal to generate the output voltage.