The invention discloses an image processing apparatus for improving the definition of an input image. The input image consists of a plurality of pixels, and the pixels are divided into an edge portion and a background portion. The image processing apparatus includes a compound operating unit, a sharpness processing unit, and an edge detecting unit. The compound operating unit is used for generating a noise-eliminated image by subjecting the input image to an open-close process and a close-open process. The sharpness processing unit is used for generating an enhanced image by subjecting the input image to a sharpness process. The edge detecting unit is used for receiving the noise-eliminated image and the enhanced image and determining whether each pixel of the input image belongs to the edge portion or the background portion, so as to selectively output the gray level of each pixel of the enhanced or noise-eliminated image.