An image processing method and a display device thereof are provided. The method is adapted to a display panel with a display area. The image processing method includes the following steps. Whether original images to be displayed on a plurality of sub-areas of the display area are still is analyzed and determined. When the original image in a current sub-area is still, a time length of the original image being still is recorded. The current sub-area is one of the sub-areas. Processing levels corresponding to a plurality of image processing schemes are determined based on the time length. Overall or partial luminance (luma) of the original image in the current sub-area is gradually reduced by the image processing schemes with the determined processing levels, and a corresponding luma-reduced image is displayed on the current sub-area.