The present invention related to a method for retrieving a tumor contour of an image processing system that includes a memory storing a grayscale image and a processor, comprising: receiving an input tumor contour of the grayscale image; defining a tumor contour annular region and a plurality of reference segments of the grayscale image, wherein the input tumor contour is in the tumor contour annular region, and each of the plurality of reference segments is across the tumor contour annular region and includes a plurality of measured points; retrieving a tumor contour suggestion point on each of the plurality of reference segments; and linking all the tumor contour suggestion points on all of the reference segments, for forming the tumor contour. Accordingly, by applying the method of the present invention, a doctor can rapidly and accurately identify the contour of a tumor in a grayscale image.