Sonication in water reduced the average contour lengths of nanocellulose prepared from wood cellulose fiber and microcrystalline cellulose. Most of the kinks in the wood cellulose nanofibrils were formed during the initial 10 min of sonication. Fragmentation occurred at the kinks and rigid segments associated with depolymerization during subsequent sonication for 10-120 min, resulting in the formation of cellulose nanocrystals with low aspect ratios. Solid-state cross-polarization magic angle sample spinning C-13-nuclear magnetic resonance revealed that the original crystalline regions of the cellulose were partly transformed to fibril surfaces or disordered regions by both pretreatment and the subsequent fragmentation of molecular chains during sonication. The nanocellulose prepared from microcrystalline cellulose had different fragmentation behavior with regard to molecular chain length following sonication. The results indicated that on average the hexagonal 36 cellulose chain structure formed the cross-section of each wood cellulose microfibril.