A method and an image processing device for image enhancement are proposed. The method includes the following steps. An input image is received. A curve set is applied on the input image to generate a resultant image, where the curve set includes a plurality pairs of weight functions and process functions, where a summation of a multiplication of the weight function and the process function in each of the pairs is zero, and where the weight function in each of the pairs is associated with low-pass filtering. An output image is generated according to the resultant image and the input image.