Thinning is the operation that seeks to reduce a connected region of pixels of a given property set to a small size. One of the most applications of thinning is the image recognission. In this paper, we proposed a new algorithm for thinning an image using Neural Net approach called (Thinning Back- propagation algorithm, TBP), the results show that the operation considered including Zhang Suen algorithm, Hilditch's algorithm. Applying Zhang Suen and Hilditch's algorithms showed that the output of the Hilditch's algorithm is better from the presence of shape accuracy point of view. The proposed Thinning Back-propagation (TBP) algorithm is superior to other algorithms; the results show that the Hilditch's algorithm gives somewhat better results than Zhang Suen algorithm, therefore the proposed TBP has been implemented depending on the Hilditch's algorithm. The accuracy of the TBP is equivalent to Hilditch's algorithm but it is a little bit faster than Hilditch's algorithm. Experimental results are presented to compare the performance of the algorithms.