Abstract: Nano crystalline cellulose (NCC) was prepared from larch and Populus tomentosa pulp by acid hydrolysis method and ultrasound treatment in this study. The effects of α-cellulose and degree of polymerization on the yield, structure, crystallinity, and thermal stability of NCC were studied. The mass fraction of α-cellulose in larch pulp was 88.93% and the average degree of polymerization was 1048.11, while the mass fraction of α-cellulose and degree of polymerization of P. tomentosa pulp were 85.90 % and 976.07, respectively. The results showed that the acid hydrolysis affected the crystal region of cellulose inconspicuously and the amorphous region was destroyed by sulfuric acid. Crystallinity of NCC was decreased by the sonication. NCC prepared from larch precented high yield and obvious characteristic absorption peaks of the crystal region. Its thermal degradation temperature was 295.2 ℃, while that of NCC prepared from P. tomentosa was 283.1 ℃.