A method of displaying image data is provided, which includes analyzing histograms by color signals of an input image frame; confirming grayscales by color signals in a predetermined frame unit with reference to the analyzed histograms; determining dimming factors in consideration of maximum grayscale values of the grayscales; determining image gains of the image data by color signals using the determined dimming factors; and outputting an image signal by applying the image gains to the input image and applying the dimming factors to a light source.