An impedance calibration circuit may include a first reference voltage generator configured to generate a first reference voltage in response to reference voltage calibration signals, a second reference voltage generator configured to provide a second reference voltage as a conversion voltage, an impedance calibration signal generator configured to compare the conversion voltage with the first reference voltage and generate impedance calibration signals when an enable signal is activated, and a register configured to store the impedance calibration signals finally calibrated and generate reference voltage calibration signals in response to the stored impedance calibration signals.