Abstract: Contrast enhancement is a very important problem in image processing. The key issue is how to assign correct enhancement levels for the local regions in an image, which makes previous methods incur much artifacts, e.g., over-enhancement, halo. In this paper, an example-based contrast enhancement algorithm is proposed, which works in the gradient domain. We utilize GMM model to describe the gradient distribution of an image. Then a GMM-based gradient mapping method is proposed to transfer the gradient of a reference image to the source image. The enhanced image is obtained by solving a Poisson equation defined by the altered gradient. Experimental results show the effectiveness and robustness of our method.