A display device and a data compensating device for a display device are disclosed. In one aspect, the data compensating device includes a converter configured to convert image data corresponding to a plurality of pixels in a display panel into input grayscale data and provide the input grayscale data. A brightness ratio calculator is configured to calculate a brightness ratio of the input grayscale data for each pixel based on a predetermined gamma value and a maximum grayscale data. The data compensating device also includes a compensating coefficient output unit configured to output a predetermined compensating coefficient based on the brightness ratio, and a compensating data calculator configured to calculate compensating grayscale data corresponding to all of the grayscale data based on the compensating coefficient. A data output unit is configured to provide the compensating grayscale data corresponding to the input grayscale data as output grayscale data to the pixels.