Abstract: The changes in ultrastructure and crystallinity of the heartwood of Cun-ninghamia lanceolaia, Catalpa bungei and the sapwood of Populus deltoides during decay by C. versicolor and G. trabtum were studied by Scanning Electron Microscopy and X-ray diffraction method. Results show that Populus dehoidcs was havily rotted by the white-rot and brown-rot fungi. The crystallinity was decreased by 50% after 12 weeks decay by the brown-rot fungus. But treated wity the same methods and duration only slight change were found in the ultrastructure and crystallinity of Cunninghamia lanceolaia and Catalpta bungei, indicating that the heartwocd of the two species are very resistant to both C. versicolor and G. trabeum.