Abstract: Compositing is the most common form of digital image forging. For a class of JPEG image forgery, a simple and effective detecting algorithm is proposed on the basis of inconsistencies of blocking artifacts between tampered region and non-tampered region. Firstly, a testing image is cropped and recompressed with the estimated primary quality factor. Secondly, blocking artifacts factor map is extracted via computing the distortion caused by recompression. Finally, automatic detection and location of tampered region is realized by using image segmentation. Experimental results demonstrate that the proposed approach is applicable to images of various qualities and lesser-tampered regions. When the difference of quality factors between double compression and single compression is greater than 15, the accuracy reaches above 93% with a less than 5% false positive rate.