An image-scaling-down method for scaling down a pixel block of an image component is provided. The pixel block includes a plurality of sub-blocks and each sub-block includes m number of non-processed pixel data where m is a positive integer. The image-scaling-down method comprises: determining a scaling down factor wherein m divided by n is the scaling down factor and n is a positive integer less than m; and calculating n number of processed pixel data in each sub-block by using the scaling down factor and the m number of the non-processed pixel data, wherein the combination of the n number of the processed pixel data of the sub-blocks corresponds to the image component.