An organic light emitting display and a driving method thereof. The organic light emitting display includes a display unit for emitting light in response to a current flowing through the display unit from a first power supply to a second power supply. The current corresponds to a data signal and a scan signal. According to one embodiment, the organic light emitting display further includes a power supply unit having a first output terminal for outputting a first power of the first power supply and a second output terminal for outputting a second power of the second power supply to the display unit, and a driving voltage calculation unit for determining a voltage of the second power corresponding to the current, thereby the power consumption of the organic light emitting display may be reduced.