Brazil is the largest producer of sugar cane bagasse in the world and the combination of this biomass with oxy-fuel combustion has become a promising technology from environmental and energetic points of view. Fundamental knowledge on the thermal behavior of biomasses is important in the development and optimization of efficient processes. This study compares the effects of both oxy-fuel (CO2/O2) and conventional (N2/O2) atmospheres of combustions on the thermal decomposition of sugar cane bagasse. An electrically heated Drop Tube Furnace (DTF) of 60 X 400 mm internal dimensions and a thermogravimetric analyser (TG) were used to evaluate the thermal decomposition yield and the activationenergy (Ea) of the bagasse. Thermogravimetric tests using a sample mass of 7.0 ± 0.5 mg were performed. Three heating rates (10, 15 and 25 °C·min-1) from room temperature up to 650 °C and a “Model Free” isoconversional model were used to determine the activation energy for the combustion of sugarcane bagasse under both atmospheres. The results show that when N2 is replaced by CO2, the activation energy value increases on average 90 %. From DTF, the TG experiments of ashes showed that the highest yield was obtained under CO2/O2 environment.