An organic light emitting display device is provided. The device includes a photo sensor adapted to: sense a brightness of ambient light; output a pulse width of an emission control signal corresponding to a sensed brightness of the ambient light; and output a gamma compensation coefficient corresponding to the sensed brightness of the ambient light and a user selected brightness. The device also includes a gamma compensation circuit adapted to adjust a magnitude of a voltage between a plurality of gradation voltages according to the output gamma compensation coefficient. The device also includes a scan driver and a data driver. The device also includes a pixel portion including a pixel adapted to: emit light according to the data signal, the scan signal, and the emission control signal; and display an image corresponding to the user selected brightness.