Abstract: In this paper the gas permeability of wet Eucalyptus urophylla×E.grandis wood was measured, drying tests were carried in several drying conditions and the collapse rule was observed, then the effect of the permeability of E.urophylla×E.grandis wood on collapse in drying was discussed. The test results showed the region of the easy collapse in transition from sapwood to heartwood, and the sapwood permeability was larger than the heartwood and the permeability decreased quickly from sapwood to heartwood. When the relative humidity was constant, the higher the temperature was, the greater the depth of shrinkage was. When the temperature was constant, the lower the relative humidity was, the greater the depth of shrinkage was. The small permeability in heartwood mainly was due to there were large tylosis in the vessels and vestured pit between the vessels. The reason of collapse in the wood was actions result of the compressing stress generated interior during wood drying and the smaller permeability caused greater capillary tension and the vapor partial pressure difference.