A delay lock loop including a selection unit, a delay unit, and a phase detection unit is provided. The selection unit receives a non-inverted clock signal and an inverted clock signal and generates a first clock signal and a second clock signal according to an indication signal. The delay unit is coupled to the selection unit. The delay unit includes a delay factor and delays the first clock signal to generate a third clock signal according to the delay factor. The phase detection unit is coupled to the delay unit and the selection unit and generates the indication signal according to a phase difference between the second and third clock signals. The delay unit adjusts the delay factor according to the indication signal.