An apparatus for encoding an image data includes a sub-pixel rendering unit configured to convert a first image data of a RGB type, supplied from an outside thereof, into a second image data of a RG-BG type by performing sub-pixel rendering on the first image data, a first differential pulse code modulation (DPCM) processing unit configured to generate a first differential data including a differential value between gray scale values corresponding to green sub-pixels in the second image data, and a second DPCM processing unit configured to generate a second differential data including a differential value between a gray scale value corresponding to a red or blue sub-pixel in the second image data and an average value of gray scale values corresponding to green sub-pixels adjacent to the red or blue sub-pixel.