During the process of automatic image recognition or automatic rnreverse design of IC, people often encounter the problem that some sub-images must rnbe pieced together into a whole image. In the traditional piecing algorithm for subrnimages, a large accumulated error will be made. In this paper, a relaxation algorithm rnof piecing-error for sub-images is presented. It can eliminate the accumulated error rnin the traditional algorithm and greatly improve the quality of pieced image. Based rnon an initial pieced image, one can continuously adjust the center of every sub-image rnand its angle to lessen the error between the adjacent sub-images, so the quality of rnpieced image can be improved. The presented results indicate that the proposed rnalgorithm can dramatically decrease the error while the quality of ultimate pieced rnimage is still acceptable. The time complexity of this algorithm is O(n In n).