An output driver for use in a semiconductor is capable of maintaining its slew rate constantly regardless of PVT (Process/Voltage/Temperature) variation. The output driver includes a pre-driving unit for pre-driving a data signal; a main driving unit for driving an output pad in response to the output signal of the pre-driving unit; and a slew rate modeling unit for generating a pre-driver bias signal to constantly maintain effective resistances of a pull-up path and a pull-down path of the pre-driving unit by modeling the pre-driving unit.