This study investigated variations in the chemical compositions and thermal decomposition kinetics of Japanese cedar and beech wood during heat treatment. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectra revealed that various reactions, such as hemicellulose deacetylation, condensation reactions causing lignin cross-linking, and reductions in cellulose amorphous regions, were carried out during the heat treatment process. In addition, combinations of FTIR spectra and principal component analysis (PCA) succeeded in discriminating the major changes in functional groups of wood at various heat treatment temperatures. On the other hand, the decrease in the storage modulus of wood was more rapid in the presence of oxygen than in an oxygen-free atmosphere during heat treatment. Accordingly, the activation energies of thermal decomposition using Arrhenius model for Japanese cedar and beech were 120.6 and 141.3 kJ/mol under air, respectively, while these values were 124.8 and 150.0 kJ/mol under nitrogen, respectively. •This study succeeded in discriminating the major changes in functional groups of wood at various heat treatment temperatures.•The thermal decomposition kinetics of wood were measured in single cantilever bending mode by DMA.•Little change was observed in E′ as a function of time at temperatures below 140 °C.•The activation energy of thermal decomposition for Japanese cedar was higher than that for beech.