Alkali treatment can change the structures and properties of cellulosic fibers. The aim of this work was to study the mechanism of structural changes of Newcell fibers treated with different alkali concentrations and two treatment methods. Raman spectra showed that the molecular conformation of Newcell fibers remained unchanged. X-ray diffraction indicated that the crystal structure of Newcell cellulose II, treated with different alkali concentrations and different methods, did not change. With the increase of alkali concentration the crystallinty and crystallinity orientation index of Newcell fibers in their original length decreased slightly, whereas those of fibers in relaxed condition substantially decreased, and the crystallite size of 101 and 002 increased in both methods. The quasi-crystallite disassociation and recrystallization in the quasi-crystalline phase, during the process of alkali treatment, led to changes of crystallinity and orientation index of Newcell fibers. (C) 2004 Wiley Periodicals, Inc.