A data driver and an organic light emitting display device including the data driver. The data driver including a shift register part to generate sampling signals in sequence, a latch part to store external data in response to the sampling signals, a current digital-analog converter to generate a data current corresponding to the data stored in the latch part, and a comparator producing a current corresponding to the difference between a pixel current and the data current. The comparator compares the pixel current fed back from each pixel with the data current, controls the data voltage by increasing or decreasing the current output, and supplies the controlled data voltage to the pixel. By controlling the data voltage an image is displayed with desired brightness regardless of non-uniformity of transistors in each pixel.