An input/output (I/O) circuit may be provided. The I/O circuit may include an input control circuit and an output control circuit. The input control circuit may be configured to apply a stress to a transmission path based on an input signal while in a test mode and buffer the input signal using a drivability changed by the stress applied to the transmission path to generate first and second transmission signals while in a normal mode after the test mode. The output control circuit may be configured to drive and output an output signal according to the first and second transmission signals based on a test mode signal.