An input output device coupled between a core circuit and a pad and including an output cell, an input cell, and a pre-driver. The output cell includes an output stage and a voltage level converter. The output stage includes a first transistor and a second transistor connected to the first transistor in serial between a first supply voltage and a second voltage. The voltage level converter generates a first gate voltage to the first transistor according to the first voltage and a data signal. When the first supply voltage is increased, the first gate voltage is increased. When the data signal is at a high level, the first transistor is turned on. The input cell includes a pull unit and a first N-type transistor. The pre-driver turns off the first and the second transistors.