Abstract: The drying defects，such as splits，checks and collapses，are developed during the drying of Eucalyptus urophylla×E.grandis. In order to improve the dried wood quality by reducing those drying defects，the reasons that caused the collapse was explored. The effect of two pretreatments，i.e.，microwave and freezing，on wood permeability，drying rate and characteristics of collapse，were investigated. The results indicated that the permeability was more or less increased，the drying rate was raised and the collapse was reduced by using these treatments. On the control and microwave treated wood，it was observed that the crumples or wrinkles took place on borders of heartwood and sapwood. It was also seen that some wood fibers were twisted and squashed each other，their pit chambers were narrowed and the twist bands which vicinity squashed and collapsed in opposite directions were formed. The width of the twist band included 5—20 cells and the depth of the collapse was 0.76 mm on the control wood. While the width of the twist band included 5—10 cells and the depth of the collapse was 0.35 mm on the microwave treated wood. No considerable collapses were found and twist bands with a width of 1—2 cells were observed vaguely on the freezing treated wood.