This study investigated a new heat treatment process using supercritical carbon dioxide. Specimens were prepared from sugi heartwood. The specimens conditioned at three different moisture contents (0%, 11%, and 17%), were heated in supercritical carbon dioxide. As a result, weight loss and anti-swelling efficiency (ASE) increased with increasing treatment temperature and specimen moisture content. The maximum ASE of the heat-treated specimens reached approximately 70%. In contrast, the equilibrium moisture content (EMC) decreased with increasing treatment temperature and specimen moisture content. The results suggest that higher treatment temperature and moisture content may accelerate the thermolysis of wood components, especially hemicellulose, leading to a decrease in adsorption sites. It was also found that the wood specimens heat-treated in supercritical carbon dioxide have higher dimensional stability compared with those prepared by other conventional heat-treatment processes. In the case of heat-treatment under high pressure, it is thought that the water molecules in the specimen might be given high reactivity to accelerate the thermolysis of wood components.