A phase interpolator includes a current generating circuit, a current controlling circuit and a signal generating circuit, wherein the current generating circuit is arranged to generate a current; and the current controlling circuit is arranged to generate a control signal to the current generating circuit to control a current value of the current. The signal generating circuit includes a capacitor, wherein the signal generating circuit generates a phase interpolation signal by using the capacitor to receive the current, wherein a phase of the phase interpolation signal is varied according to the current.