<p>Wood is attracting attention as an environmentally friendly resource that does not burden the environment and is indispensable for the realization of a low-carbon society. In the field of construction, the use of heat-treated wood has been gaining traction in recent years for the purpose of improving dimensional stability and decay resistance. However, there have been few studies on the non-destructive measurement of the strength of heat-treated wood. Only cellulose, which accounts for about half of the major chemical components of wood, has a crystalline form, and changes in the interior of wood due to thermal treatment are thought to result from changes in this crystalline structure. Terahertz time-domain spectroscopy (THz-TDS) is suitable for crystallographic studies as the radiation energy in the THz region (0.1–10 THz; wave number 3.3–333 cm⁻¹) corresponds to lattice vibrations (phonons) of cellulose crystals. In this study, THz-TDS was used to evaluate the crystallinity of heat-treated cellulose and heat-treated wood powder. Wood pellets were measured by THz-TDS and X-ray diffraction (XRD) before and after the thermal treatment. The absorption peaks and XRD patterns were compared, and the effects of varying the temperature and duration of thermal treatment were investigated. Results indicated that crystallinity decreased rapidly at 250&#xa0;°C, while at 200&#xa0;°C, the reduction was more gradual, with signs of recrystallization. These findings suggest that factors such as temperature, humidity, and removal of non-crystalline components (e.g., hemicellulose, amorphous cellulose) are key in determining crystallinity during thermal treatment of wood.</p>