Principle of nonlinear correction for radiographic image by optic density vs. path length curve was introduced. By the principle of radiography, the function of the optic density and the path length was monotonous on conditional that the X-ray radiation and the scatter distribution were regular. So the optic density vs. path length curve could be used to correct the radiographic image. A background was added on the corrected image when the scatter distribution was non-regular. A radiographic image of triangle object was corrected using this method, and the results confirmed the method.