An input/output (IO) circuit with high voltage tolerance is provided. In an integrated circuit, the IO circuit includes a charge pump for generating a bias voltage higher than an internal operating voltage of the charge pump itself, and a switch between an external circuit and an internal circuit of the integrated circuit. When the switch conducts between the external circuit and the internal circuit, the switch provides a clamping voltage according to the bias voltage and a cross voltage of the switch, so that a voltage of the internal circuit is bounded by the clamping voltage to prevent the internal circuit from over-voltage.