A display device receiving a video signal including grayscale data and displaying images according to the input video signal includes: a photosensor measuring illuminance of external light; a timing controller processing the data signal according to a target gamma value corresponding to the measured illuminance among a plurality of gamma values for a reference curved line representing a relationship of a lightness variation according to a change of grayscale data to be applied to the measured illuminance environment; a data driver applying the processed data signal to a plurality of pixels; and a scan driver applying a scan signal to the plurality of pixels for the data signal to be applied to the plurality of pixels. Accordingly, an image or a motion picture may be displayed with a desired grayscale without influence of external light, and the display quality may be obtained in an external light environment without increasing power consumption.