A kind of spherical cellulose nanocrystals was prepared by hydrolysis of microcrystalline cellulose with mixed acid. In order to improve the thermal stability, two methods were used: diminishing the acid sulfate groups by desulfation and neutralizing them by using NaOH solution. The thermal degradation behaviors were characterized by thermogravimetric analysis and differential scanning calorimetry at nitrogen current. The results indicated that the degradation of cellulose nanocrystals with acid sulfate groups started at lower temperature and showed two remarkable pyrolysis processes. Profile analysis of the derivative thermogravimetric curves showed that each pyrolysis process was composed of multi-step reactions. When neutralized by NaOH solution, the degradation shifted to the higher temperature and occurred within a narrow temperature range. The influence of particle size of cellulose on degradation was also investigated. The results indicated that the degradation of cellulose with small size took place at lower temperature and facilitated the char residue formation. (C) 2007 Published by Elsevier Ltd.