The ultrastructure of eucalyptus kraft pulp fibers was studied by using solid-state cross polarisation magic angle spinning carbon-13 nuclear magnetic resonance (CP/MAS 13C NMR) in combination with spectral fitting. Their NMR spectra were analyzed in detail and some structure parameters such as the cellulose crystallinity, allomorph composition, relative amounts of different cellulose allomorphs present and lateral dimensions for cellulose elementary fibrils and microfibrils were determined. The results show that the relative contents of cellulose Iα allomorph and cellulose Iβ allomorph obtained by the spectral fitting for the cellulose C1-region (δ 102～108) are 26.92% and 52.04%, respectively. The cellulose crystallinity calculated by the NMR method was 47%. The lateral dimensions for cellulose elementary fibrils and microfibrils obtained by the spectral fitting for the cellulose C4-region (δ 80～92) were 4.0 and 17.9 nm, respectively. The cellulose crystallinity calculated by relative amounts of different cellulose allomorphs was 51%. It has been proved that the para-crystalline cellulose has a well-ordered “in-core” structure.

Key words: CP/MAS ¹³C NMR, cellulose, crystalline structure, elementary fibril