The disclosure relates to data driver and organic light emitting display device. The data driver includes: an input unit configured to receive an input data; a compensation data generator configured to generate a compensation data by applying a compensation value to the input data; a converter unit configured to convert the input data into an image data voltage and to convert the compensation data into a compensation data voltage; and an output unit configured to separately output the image data voltage and the compensation data voltage to a data line of the organic light emitting display.