Disclosed is an open loop fractional frequency divider including an integer divider, a control circuit, and a phase interpolator. The integer divider processes an input clock according to the setting of a target frequency to generate a first frequency-divided clock and a second frequency-divided clock. The control circuit generates a coarse-tune control signal and a fine-tune control signal according to the setting. The phase interpolator generates an output clock according to the first frequency-divided clock, the second frequency-divided clock, and the two control signals. The two control signals are used for determining a first current, and their reversed signals are used for determining a second current. The phase interpolator controls a contribution of the first (second) frequency-divided clock to the generation of the output clock according to the first (second) frequency-divided clock, the reversed signal of the first (second) frequency-divided clock, and the first (second) current.