Nanocrystalline cellulose (NCC) was prepared from microcrystalline cellulose (MCC) by acid hydrolysis．It was observed that the diameter of NCC particles mainly distributed over 30～50 nm by transmission electron microscope (TEM). The crystal form and degree of crystallinity were detected by X-Ray diffraction．The results showed that NCC and MCC have the same crystal form of cellulose Ⅰ

and that the reactions mainly occurred in the amorphous region of MCC during the acid hydrolysis process．The thermal behavior of NCC in different pH conditions was characterized by differential scanning calorimetry (DSC)．The consequences indicate that the thermal stability of NCC distinctly decreased by contrast with the thermal stability of MCC

and that the thermal stability of NCCs in alkali conditions was higher than that of NCCs in acid conditions．The specific surface area distinctly increased with sharp decreasing of the particle size of NCC．This induced the end carbons and active groups of surface of NCC to increase sharply

therefore caused the thermal stability of NCC to distinctly abate．That NCC has very strong adsorption affinity is the cause of the obvious difference of thermal behavior of NCC in different pH．In acid conditions the surface of NCC adsorbed a great lot of H+

which induced the cellulose chains of surface of NCC to decompose at low temperature catalyzed by H+．When adding sodium hydroxide solution

on one hand the H+ of surface of NCC was neutralized；on the other hand

the cellulose chains of low molecular weight were dissolved in sodium hydroxide solution and the defects of surface of NCC rearranged and stable structure formed．These factors improved the thermal stability of NCC in alkali conditions．