An organic light emitting display device, comprising: a pixel unit comprising a plurality of pixels, each pixel comprising an organic light emitting diode (OLED) and a pixel circuit coupled to a power line (310, 320), wherein an anode of the organic light emitting diode (OLED) is coupled to the pixel circuit; a first voltage supply unit configured to output a first voltage (ELVDD1) to a first voltage (ELVDD1) output; a second voltage supply unit configured to output a second voltage (ELVDD2); a third voltage supply unit connected to a cathode of the organic light emitting diode (OLED) and configured to output a third voltage (ELVSS); and a selection unit configured to select one of the first and second voltages (ELVDD1, ELVDD2) and to supply the selected voltage to the pixel circuits through the power lines (310, 320), wherein the selection unit is configured to supply the first voltage (ELVDD1) during a first period and to supply the second voltage (ELVDD2) during a second period, the first and second periods (P1, P2) being alternately repeated, and wherein the pixels are configured to emit light during at least a partial period of the first period and to perform an initialization or compensation operation while in a non-emission state during at least a partial period of the second period, characterized in that the organic light emitting display device further comprises a voltage controller configured to supply first and second control signals (Cv1, Cv2) to the first voltage supply unit, wherein the first voltage supply unit is adapted to output the first voltage (ELVDD1) having a first voltage level when receiving the first control signal and to output the first voltage (ELVDD1) having a second voltage level when receiving the second control signal, wherein the voltage controller is further adapted to supply a first compensation signal corresponding to a variation between the first voltage (ELVDD1) and a first reference voltage (Vref1) to the first voltage supply unit, wherein the voltage controller is adapted to measure a first measurement voltage level from the first voltage (ELVDD1) output upon receiving the first control signal from the first voltage supply unit and to measure a second measurement voltage level from the first voltage (ELVDD1) output upon receiving the second control signal from the first voltage supply unit and the voltage controller is further adapted to generate a linear function passing through a first coordinate composed of the first voltage level and the first measurement voltage level and passing through a second coordinate composed of the second voltage level and the second measurement voltage level, wherein the voltage controller is further adapted to calculate a setup voltage level (Vs) corresponding to a predetermined target voltage level (Vt) using the produced linear function and to set the first reference voltage (Vref1) to the produced setup voltage level (Vs). a pixel unit comprising a plurality of pixels, each pixel comprising an organic light emitting diode (OLED) and a pixel circuit coupled to a power line (310, 320), wherein an anode of the organic light emitting diode (OLED) is coupled to the pixel circuit; a first voltage supply unit configured to output a first voltage (ELVDD1) to a first voltage (ELVDD1) output; a second voltage supply unit configured to output a second voltage (ELVDD2); a third voltage supply unit connected to a cathode of the organic light emitting diode (OLED) and configured to output a third voltage (ELVSS); and a selection unit configured to select one of the first and second voltages (ELVDD1, ELVDD2) and to supply the selected voltage to the pixel circuits through the power lines (310, 320), wherein the selection unit is configured to supply the first voltage (ELVDD1) during a first period and to supply the second voltage (ELVDD2) during a second period, the first and second periods (P1, P2) being alternately repeated, and wherein the pixels are configured to emit light during at least a partial period of the first period and to perform an initialization or compensation operation while in a non-emission state during at least a partial period of the second period, characterized in that the organic light emitting display device further comprises a voltage controller configured to supply first and second control signals (Cv1, Cv2) to the first voltage supply unit, wherein the first voltage supply unit is adapted to output the first voltage (ELVDD1) having a first voltage level when receiving the first control signal and to output the first voltage (ELVDD1) having a second voltage level when receiving the second control signal, wherein the voltage controller is further adapted to supply a first compensation signal corresponding to a variation between the first voltage (ELVDD1) and a first reference voltage (Vref1) to the first voltage supply unit, wherein the voltage controller is adapted to measure a first measurement voltage level from the first voltage (ELVDD1) output upon receiving the first control signal from the first voltage supply unit and to measure a second measurement voltage level from the first voltage (ELVDD1) output upon receiving the second control signal from the first voltage supply unit and the voltage controller is further adapted to generate a linear function passing through a first coordinate composed of the first voltage level and the first measurement voltage level and passing through a second coordinate composed of the second voltage level and the second measurement voltage level,