Fabric extension can be measured by using fabric strength tester in laboratory. But the extension is difficult to be measured when the clothing is in wearing. In this paper, the relationship between the extension of clothing and the fabric tension was discussed according to the experiment result. The digital image processing will be used to grab clothing images and to measure the extension of fabric. The techniques of digital image processing, such as binary conversion of image, noise points deleting, smooth processing of stripe edge and the stripe width measuring, were applied in this research. Based on experiment results, the relation between the fabric tension and extension were linear when the clothing was under the condition of small strength.