A calibration circuit for calibrating a differential driver with a differential output port including a first output node and a second output node includes: a comparing circuit arranged to receive a first output voltage corresponding to the first output node and a second output voltage corresponding to the second output node, and generate a comparison result according to the first output voltage, the second output voltage, and a predetermined voltage; and a controlling circuit coupled to the comparing circuit, a first resistive element and a second resistive element. The controlling circuit is arranged to adjust the first resistive element and the second resistive element according to the comparison result, wherein the first resistive element is coupled between the first output node and a reference voltage, and the second resistive element is coupled between the second output node and the reference voltage.