Abstract: X-ray devices have demonstrated the ability to characterize a material at the molecular and atomic level. This ability is particularly important for detecting illicit materials. As X-ray detection methods which have increasingly been as an effective means for automatic detection of illicit materials now commercially available, most of these technologies are not yet mature. Object classification methods for using x-ray dual-energy transmission technologies more effectively was investigated., Investigation on object classification based on dual-energy x-ray transmission technology was made. R is a dimension relation to the effective atomic number. Classify the materials into organic 、inorganic and mixture by effective atomic number. It shows this method is very efficient by experimentation.