Heat treatment is an eco-friendly and efficient way to improve the defective properties of woods, such as hygroscopic nature, lack of dimensional stability, and low resistance against biological degradation, and to produce a green and sustainable wood material for construction and buildings. The aim of this study is to investigate the thermal degradation of a hardwood (poplar, Populus nigra) and a softwood (fir, Abies pectinata) in a semi-industrial scale reactor in which a vacuum environment is adopted to intensify the thermal degradation process. Four different stages of thermal degradation during wood heat treatment are defined, based on the intensity of differential mass loss (DML). Meanwhile, a number of analyses on untreated and treated woods are performed to evaluate their thermal degradation characteristics and compositional change during treatment. The FTIR analysis clearly demonstrates the thermal degradation through dehydration, deacetylation, depolymerization, and condensation reactions during the heat treatment. In addition, the XRD analysis indicates an increase in relative crystallinity of cellulose. The correlation of devolatilization index (DI) with respect to mass loss of the two wood species is strongly characterized by linear distribution, which is able to provide a simple and useful tool in predicting mass loss of wood treated in wood industry.