An impedance calibration circuit includes a first reference resistor electrically coupled to a calibration pad, a second reference resistor which is coupled to the first reference resistor in parallel and a resistance value of the second reference resistor is varied according to an operation voltage mode, and a calibration circuit electrically coupled to the calibration pad and configured to generate a calibration code according to a resistance value formed by the first reference resistor and the second reference resistor and calibrate an impedance value in the calibration pad according to the calibration code.