In a method of operating a nonvolatile memory device, an impedance calibration verifying operation is performed based on a data read command. The impedance calibration verifying operation ascertains whether an impedance calibration operation is normally performed for a data input/output (I/O) terminal of the nonvolatile memory device. A detection value is stored in a storage unit. The detection value indicates a result of the impedance calibration verifying operation. The detection value is output based on a first command received after the nonvolatile memory device receives the data read command. A data read operation or the impedance calibration operation is selectively performed based on the detection value.