Abstract: In the domain of digital image forensics, a novel detecting approach called Blind Detection has been developing rapidly. Unlike Digital Watermarking, which needs watermarks be embedded into an image beforehand, Blind Detection technology is able to detect image based only on the intrinsic features of an image. Among which the detection of image manipulating traces is one of the crucial means in digital image forensics. An algorithm of detecting the trace of blur processing is proposed in this paper. Different Gaussian radii are utilized in re-blurring an original image, according to the fundamental of Gaussian Blur in image processing. Then changes of image edge after being blurred are compared to detect the different blur features in the original image, thereby traces of blur processing can be found with our proposed algorithm. The simulation results indicate that our proposed method is valuable in Passive Blind Image Forensics.