Cellulose is an important material for production of biofuel and refined chemicals. Pyrolysis is one of the most promising approaches for cellulose de-polymerization. Understanding the mechanism of cellulose pyrolysis is essential for development of efficient biomass conversion technologies. In this study, the thermodynamic energy change of cellulose pyrolysis through homolytic bond cleavage was studied with the aid of density functional theory method by using cellulose dimer as a model compound. The free energy changes of various homolytic bond dissociation of cellulose dimer were studied by the method of M06-2x at the temperature of 800 ℃. To compare with experiment results of cellulose pyrolysis reported recently by Huber et al., the free energy changes of reaction pathways studied by Auerbach group via Car-Parrinello molecular dynamics calculations were also studied. Calculated results show that the free energy changes of homolytic dissociation of glucosidic bond varies in the range of 45～51 kcal/mol. The free energy changes of homolytic bond dissociation of C—OH bond vary in the range of 62～70 kcal/mol. The free energy changes of homolytic bond dissociation of O—H bond vary in the range of 82～95 kcal/mol. The free energy changes of homolytic bond dissociation of C—C bond vary around 46 kcal/mol. Furthermore, thermodynamic stabilities of products were found to be irrelevant to the ratios of them in Huber et al.'s fast pyrolysis experiments no matter at a high temperature (800 K) or a mild temperature (298 K). Finally, we found that temperature have a significant influence on whether a reaction can occur spontaneously or not. At relatively high temperature, the reactions having significant increase of entropy are favored, e.g. dehydration. The free energy of reactions with small entropy changes are less sensitive to the change of temperature. This foundation provides an inspiration for controlling chemical selectivity of different types of reactions by temperature regulation.