An image processing method includes at least following steps: partitioning a picture into a plurality of picture regions, wherein each picture region comprises at least one pixel group row, each pixel group row comprises at least one pixel group, and the picture regions comprise a first picture region and a second picture region horizontally adjacent to each other; and performing flatness check upon a specific pixel group in a pixel group row of the first picture region through using at least one pixel borrowed from the second picture region.