A fast skeletonization algorithm directly for gray-scale patterns is presented in this paper by simulating the procedure of water scouring the image surfaces.It is shown that this algorithm can produce connected and one-pixel-wide skeletons which are invariant to the strictly monotonic transformations of the original images and also keep the topological properties of the original image unchanged and lie in the middle lines of the patterns.The experiments on the binary images and the gray-scale images show the efficiency of the algorithm.