A display device and a controlling method thereof are provided. The display device includes a display panel, a backlight unit, and a time schedule controller. The display panel is configured to display images. Areas where afterimages of dynamic images occur are identified according to comparison results of dither values. Tailing phenomenon of the dynamic images may be relieved by transmitting backlight data without brightness to a second backlight sub-area corresponding to where the afterimages occur. Furthermore, because frequency multiplication is not applied, risks of display instability will not increase.