An organic light emitting display device which improves expression capability in a low luminance level region, wherein, when receiving a pulse width modulation value indicating a low luminance level region, the device changes the received pulse width modulation value into a pulse width modulation dimming value indicating a high luminance level region, and expresses a low luminance level region through a luminance level indicated by the pulse width modulation dimming value and a pulse width modulation dimming operation. Luminance in a low luminance level region is controlled through a pulse width modulation dimming value indicating a high luminance level region and a pulse width modulation dimming operation, so that it is possible to minutely control luminance in a low luminance level region and improve expression capability in a low luminance level region.