Abstract: Primary factors affecting determination of the optical path in radiography are investigated. For two different detect ways, the direct and the converting screen methods, these factors are the same, including the scattering effect, the angle distribution effect of the exposure, and the inverse effect of the exposure to the square of radiographic distance. In the high-energy radiography, the scatter is very serious and it is the most important factor. The scatter effect makes the measured value of optical path 20%~40% lower than its actual value. Before CT reconstruct of the radiographic image, it is necessary to reduce scatter effect from the measured optical path.