Provided are a display device and an image processing method thereof. The display device according an embodiment includes a display panel including pixels formed of red, green, blue, and white sub pixels, an image processing unit that converts a three-color input image supplied to the red, green, blue, and white sub pixels into four-color image data, calculates a color contrast ratio using a luminance weight and a hue weight by scaling luminance and hue values of the input image, and outputs an output image by considering the color contrast ratio, and a timing controller which aligns and outputs the output image from the image processing unit.