A clock generation circuit for a transmitter which transmits data according to an output clock signal is provided. The clock generation circuit include a clock generator and a phase locked loop (PLL). The clock generator generates a first clock signal. The PLL initially generates the output clock signal according to the first clock signal. When a frequency of the output clock signal generated according to the first clock signal is not within a range required for specification of the transmitter, the PLL generates the output clock signal according to a second clock signal.