Abstract: An input/output (I/O) circuit with a high I/O voltage tolerance is provided for use in conjunction with an IC device that operates with two system voltages, such as 3.3 V and 5 V. The particular circuit configuration of this I/O circuit allows it to be fabricated using the Single Gate-Oxide technology instead of the Double Gate-Oxide technology, so that the manufacturing cost can be reduced as compared to the prior art. Moreover, this I/O circuit allows an output impedance lower than that of the prior art, allowing the signal transmission speed via this I/O circuit to be increased by about 30% as compared to the prior art. It can also help eliminate the problems of poor gate oxide reliability, PN junction inversion, and PMOS leakage that otherwise occur in the prior art. Furthermore, this I/O circuit can help eliminate the DC leakage current in the input-stage circuit, so that the power consumption can be reduced compared to the prior art.