An organic light emitting display device is disclosed. One inventive aspect includes a plurality of pixels provided at a region sectioned by scan lines and data lines and an initialization power unit. The plurality of pixels are configured to control the amount of a current flowing from a first power source to a second power source through an organic light emitting diode in response to a data signal. The initialization power unit supplies initialization power to a driving transistor within each pixel circuit. The initialization power unit further controls the voltage of the initialization power supply to maintain a substantially constant voltage difference between the second power source and the initialization power.