Cellulose was treated at 423-623 K under hydrothermal conditions without a catalyst to produce charcoal. The charcoal generated under hydrothermal conditions was mainly analyzed by FT-IR in order to investigate changes in its chemical structures occurring during the carbonization process. Increasing reaction temperatures decreased charcoal yield, increased its carbon-to-oxygen ratio, and promoted cellulose carbonization. Furthermore, increasing reaction temperature during hydrothermal treatment of cellulose weakened FT-IR peaks from -OH and -CH groups, showing that dehydrogenation and deoxygenation had occurred. These studies confirmed that hydrothermal conditions promoted carbonization of cellulose.