Abstract: Wetheartwood can be observed more or less in all living trees or logs of Populus deltoides I-69, in their various gorwth stages no matter how they planted; in Panjiawan,Jiayu County, Hubei Province. The percentage of wetheartwood of a tree is relevant to the water content of soil, but not to the planting method or the growth rate of the tree. The occurrence and discolouration of wetheartwood may be caused by bacteria activity. Five species of bacteria (Erwinia carotovora subsp. carotovora, E. rhapontioi (Millard) Burkhold, E. chrysanthemi Burkholder, Bacillus cereus Frankland, B. sphaericus Neidehave) have been isolated and identified. Observation of electron microscopy and the wood properties analysis indicated that the pit membranes of vessel-ray pits and the intervessel pits in wetheartwood were almost destroyed by bacteria. Comparing with the normal wood, the pH and acid buffering capacity, the content of extractives and ash are all higher, but the content of holo-cellulose and a-cellulose are a little lower. Besides, the specific gravity, shrinkage and mechnical properties of wetheartwood are lower than thos of normal wood. It is suggested that the approach of preventing the occurrence of wetheartwood should be further studied.