An image processor includes an average luminance calculator which calculates an average luminance of a current frame based on a first average luminance of input image data corresponding to a partial screen and a second average luminance of previous image data corresponding to an entire screen, a conversion curve generator which generates a luminance conversion curve based on the average luminance, a luminance converter which generates conversion image data by converting luminance data of the input image data based on the luminance conversion curve, and an image merger which generates output image data by merging the conversion image data and the previous image data.