Abstract: Two kinds of fast-growing wood fibers, bleached sulphite pulp from Pinus massoniana Lamb and bleached sulfate pulp from E·urophylla, were treated with ultrasonic. The changes in morphology and supermolecular structure of the treated cellulose fiber were characterized by scanning electronic microscopy, X-ray diffraction and FTIR-spectra. The accessibility of the fibers determined as water retention value(WRV) was also discused. The results showed that significant changes occurred in the morphology of the ultrasonic treated cellulose fibers. These changes were: crack of cell wall, dislocation in the secondary wall middle layer(S₂), exposure and fibrillation of the S₂ layer. The results from FTIR and X-ray diffraction showed that the crystalline type of cellulose and the crystalline size has not changed during ultrasonic treatment, and the crystallinity changed according to the treatment conditions. The increased WRV of the treated cellulose fibers lead to an increase in accessibility and bonding force between the fibers.