An organic light emitting diode (OLED) display device and a method of fabricating the same are disclosed. The OLED display device includes a plurality of scan lines, a plurality of data lines, and a plurality of pixels disposed in a region in which the scan lines cross the data lines, where each pixel of the plurality of pixels includes: a switching transistor including a first gate electrode, a first semiconductor layer disposed over the first gate electrode, a first gate insulating layer interposed between the first gate electrode and the first semiconductor layer, a first source electrode and a first drain electrode, a driving transistor including a second semiconductor layer, a second gate electrode disposed over the second semiconductor layer, a second gate insulating layer interposed between the second gate electrode and the second semiconductor layer, a second source electrode and a second drain electrode, and an organic light emitting diode electrically connected with the second source and second drain electrodes of the driving transistor, where the first and second semiconductor layers are formed of the same material, and from the same processing.