Abstract: Based on Lambert illumination model, blind identification algorithm for image authenticity is proposed. For an image, there is an error function between its actual light intensity and calculated light intensity; and for different light source models, there are different constraint functions of light source to light beam. Therefore, for infinite light source images and local light source images, a corresponding algorithm is presented, which calculates the light source direction of different areas in an image according to the error function and the corresponding constraint function. And the image tampering can be determined by the inconsistency in light source direction of different areas in an image. That is to say, if light source directions of different areas in an image are inconsistent, it can be considered that the image has been tampered; on the other hand, if light source directions of different areas in an image are consistent, it can be considered that the image has not been tampered. This has realized blind identification for image authenticity. Experimental results show that the light source direction of different areas in an image could be calculated accurately by the proposed corresponding algorithms for the two kinds of images mentioned above. Thus, the image authenticity could be identified effectively.