A definite proportional limit which has been observed typically in metallic materials during the transition from elastic deformation into plastic one was also detected in the load-displacement curve of the compression wood with the water content about 100% from Cryptomeria japonica D. Don. Time-resolved small-angle X-ray scattering studies demonstrated a large structural change, that is, a strong decrease in the microfibril angle in the cell wall occurring with increasing the displacement beyond the proportional limit for the compression wood loaded in uniaxial tension. Correspondingly, the mechanical properties are changed and high elongations begin to be seen. For the dried normal wood, on the other hand, only a weak decrease in the microfibril angle was observed with increasing the elongation until the fracture is initiated, where the elastic behavior was maintained. (C) 2005 Elsevier Inc. All rights reserved.