Theoretical and experimental approaches for edge enhancement effect of vision on an X-ray radiographic image were carried out. Psychophysically enhanced portions in density unit at the both sides of a step edge image were derived from MTF's of vision and radiographic systems. Experimantal values were obtained using our new method. Experimental values distributed around theoretical values, and their qualitative behaviour was similar to that of theoretical values. Discrepancy between theoretical and experimental results dues to nonlinearity of human visual system.