Co-combustion with wood has been proposed to reduce carbon dioxide emissions from coal-fired power generation. Torrefaction is attracting attention as a technique for converting wood into fuel suitable for co-firing power generation. Torrefaction is a thermal reaction in an inert atmosphere at a relatively low temperature of about 200 to 300 ℃, improving the crushability of wood. In this study, we performed the compositional sugar analysis of two kinds of hardwood pellets and cellulose torrefied at different temperatures, and tracked changes in the chemical composition. Most of the xylan was lost at 250 ℃, and the cellulose content decreased with increasing temperature thereafter. Instead, chemical compounds which was not solubilized by a treatment with 72 %–4 % sulfuric acid (analytical acid-insoluble lignin) increased. It is considered that at 250 ℃ xylan was converted to acid-insoluble product and over 250 ℃, cellulose was gradually carbonized to contribute to the increase of acid-insoluble lignin. These carbonized products contribute to the improvement of the heating value of torrefied wood.