Abstract: Nanocrystalline cellulose (NCC) was prepared by ultrasonicassisted hydrolysis of microcrystalline cellulose (MCC) with carbonbased phosphotungstic acid as the catalyst. The morphological, crystal structure, spectrum properties and thermal stability properties of NCC were respectively analyzed by transmission electron microscopy (TEM), Xray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), and thermographic analyzer (TGA). The results indicated that NCC was rodlike with diameter ranging around 10-100nm and length ranging between 200-800nm. The XRD pattern showed that the NCC still kept with the typeⅠcellulose structure, and its crystallinity was 76.1%. The FTIR analysis showed that NCC was still with the basic chemical structure of cellulose. The results of TGA analysis demonstrated that the thermal decomposition temperature of NCC was 300℃, whose primary thermal stability of NCC was lower than that of MCC. Compared with the conventional acid hydrolysis, the new preparation approach of NCC could avoid the acid removing process, and it possessed the advantages of low corrosion to equipments, and less pollution to the environment and the like.