Thermogravimetric analysis (TGA) has been frequently utilized to evaluate the thermal degradation reactivity of wood and the component polysaccharides. Softwood and hardwood species are clearly discriminated by TGA; hardwood exhibits a clear low-temperature-shoulder along with a peak in derivative of TGA (DTG) curve, whereas the softwood DTG curves do not show such a shoulder. Such difference is believed to arise from the different hemicellulose composition by comparing with the DTG curves of the isolated polysaccharides, but no evidence has been reported. In wood cell walls, the constituents have a nano-multilayer structure, in which cellulose microfibrils are surrounded by the matrix of hemicellulose and lignin. Such nano-structure may affect the thermal degradation reactivity of cellulose and hemicellulose in wood. In this study, thermal reactivity of cellulose and hemicellulose in wood was directly evaluated by quantifying the amounts of cellulose and hemicellulose remaining in the heat-treated Japanese cedar (Cryptomeria japonica, a softwood) and Japanese beech (Fagus crenata, a hardwood). As a result, thermal degradation reactivity of cellulose and hemicellulose in wood was found to be quite different from the isolated polysaccharides, and the influences were different depending on the wood type. Finally, the different TG/DTG curves of softwood and hardwood species are clearly explained by the thermal degradation reactivity of cellulose and hemicellulose.