•Hemicelluloses content in the range of 9.7–21.7 wt.% led to CNFs with average diameter.•CNFs shows good thermal stability when hemicelluloses content is low.•A decrease in hemicelluloses will improve the thermal stability of the CNFs. In this work, cellulose nanofibrils (CNFs) are produced from bagasse pulps with differing hemicelluloses contents by ultrafine grinding and high-pressure homogenization. The results showed that hemicelluloses content in the range of 9.7–21.7 wt.% led to nanofibrils with average diameter. A decrease in hemicelluloses content can enhance the crystallinity and improve the thermal stability of the CNFs. The activation energy of the CNF samples with hemicelluloses contents of 9.7 wt.%, 12.72 wt.%, 15.7 wt.%, 18.76 wt.%, and 21.7 wt.% are 713.03, 518.93, 462.62, 421.78, and 211.11 kJ/mol, respectively, when the conversion rate is increased from 30%–90%. These results demonstrate that hemicelluloses content has a considerable influence on the properties of CNFs. This work provides a theoretical basis for high-value utilization of CNFs, and enriches useful information on the application of CNF materials.