Abstract: The paper analytically discusses two factors, the spectrum of photon and the scattered radiation by the detector, which affect the attenuation efficient of the object in flash Xray radiography. The effective attenuation efficient for the object components is evaluated simulating Xphoton transport process in radiographic system using Monte Carlo method. The effective attenuation efficient and its distribution on the image are obtained regardless of scattered effect. The distribution shape is similar to the CT number profile in medical CT reconstruction. The larger the optical path of the photon projecting an image, the smaller the effective attenuation efficient corresponding to the projecting point in the image. It is shown that the average effective attenuation efficient is 0.830cm-1