Abstract:The ultrastructure of the wood from the thinned Populus dividiana trees was studied by means of seanning electronmicroscope. Occurrence of tylosis was found with less amount in heartwood as well as in the transitional zone boundary from sapwood to heartwood. This might be one of the major causes of collapse in the process of wood drying. The anatomical features of collapsed cells and the ultrastructure variance after steaming were also studied in order to justify that collapse could be eliminated by steaming treatment.