An exemplary embodiment of the present disclosure illustrates a duty cycle correction apparatus for fast adjusting internal clocks to have specific duty cycles. Firstly, a reference clock is adjusted to have one specific duty cycle in response to analog feedback clocks. Then, by using a phase detector, phases of the reference clock and one internal clock are compared to generate a phase detection signal. Next, by using a digital-analog converter, complementary signals are generated according to a phase detection signal received by the counter, and the signals are used to adjust the duty cycles of the internal clocks. When the complementary signals make the duty cycle of the internal clock equals to the specific duty cycle, codes of the complementary signals are recorded.