High-intensity ultrasonication with a batch process was used to isolate fibrils from several cellulose sources, and a mixture of microscale and nanoscale fibrils was obtained. The geometrical characteristics of the fibrils were investigated with polarized light microscopy, scanning electron microscopy, and atomic force microscopy. The results show that small fibrils with diameters ranging from about 30 nm to several micrometers were peeled from the fibers. Some fibrils were isolated from the fibers, whereas some were still on the fiber surfaces. The lengths of untreated and treated cellulose fibers were investigated by a fiber size analyzer. The crystallinities of some cellulose fibers were evaluated by wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. The high-intensity ultrasonication technique is an environmentally benign method and a simplified process that conducts fiber isolation and chemical modification simultaneously and helps significantly reduce the production cost of cellulose nanofibers; and their composites. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2756-2762, 2010