The surface layer of the lumber can be greatly plasticized in the high temperature drying. Th e internal cracking may be produced due to the restraint of regular shrinkage in the internal layer in the later drying period. The internal cracking is related to the tensile stress of the core layer. When the tensile stress of the core layer is bigger than the tensile strength perpendicular to grain of wood, the internal crack will appear. Therefore, when the high temperature drying technology is used to rapidly dry the soft broadleaf lumber such as the poplar( Populus deltoides Bartr.) wood, attention should be paid to the drying defect due to the drying stress reversal in the later drying period.