Self-heating and spontaneous combustion of torrefied biomass are direct hazards related to torrefied biomass during production, storage and transportation. Thermogravimetry and differential scanning calorimetry (TGA/DSC) was used to access the effect of air oxidation combined with water sorption on self-heating of torrefied eucalyptus at 60 °C, 120 °C and 180 °C cooling temperature by measuring weight change and heat flow. The results showed that the weight of the torrefied biomass increased rapidly as soon as the gas stream was changed from nitrogen to humid nitrogen accompanying the heat generation at 60°C demonstrating the effect of water adsorption dominated. Conversely, the mass of the torrefied biomass decreased rapidly when it was exposed to dry air accompanying the heat generation at 180°C which showed the effect of air oxidation dominated. The effect of air oxidation increased with the increase in temperature while the effect of water adsorption decreased with the increase in cooling temperature. Thus, the important role of the air oxidation and water adsorption on the self-heating of torrefied biomass was clarified.