There is disclosed an image resolution adjustment method comprising the steps of: performing a predetermined selection rule to select one of scaling modes based on a source resolution Vi and a display resolution Vo to generate a scaling value set for outputting a scaled image; and determining whether a smoothing process to determine whether or not the scaled image is smoothed based on application requirements, wherein the scaling modes comprises a first scaling mode, a second scaling mode and a third scaling mode to generate each of the corresponding scaling value set, respectively. The scaling value set is provided to generate replicated pixels or lines for each pixel or line of the source image during a horizontal or vertical scaling period, and the replicated pixels or lines are arranged in horizontal or vertical symmetry in relation to its central region of the source image.