Thermal behavior of biomass in torrefaction plays an important role in the operation of pretreatment. To understand the endothermic and/or exothermic characteristics of biomass in the course of torrefaction, an experimental system is conducted and two kinds of biomass (oil palm fiber and eucalyptus) are investigated. The results indicate that the thermal behavior is significantly influenced by the lignocel-lulosic composition in biomass and the torrefaction temperature. The thermal decomposition of hemi-cellulose is the dominant mechanism for oil palm fiber torrefied at 200 and 250 C, whereas the thermal degradation of cellulose is crucial when the biomass is torrefied at 300 C. Therefore, the heat of reaction of oil palm fiber increases with increasing torrefaction temperature. The torrefaction of eucalyptus is always endothermic, as a consequence of high cellulose contained in the biomass. It is less endothermic when the torrefaction temperature increases, presumably due to the char formation from cellulose thermal degradation and the exothermic lignin decomposition. As a whole, the values of the heat of reaction of the two samples are between À3.50 and 2.23 MJ/kg. The obtained results have provided a useful insight into the control of torrefaction operation and the design of torrefaction reactor.