The influence of oxygen concentration on the char burnout characteristics of the coniferous tree (beefwood) and broadleaf tree (beech), uhich are abundantly available in Vietnam under-reacting temperature of 800 oC, and 1000 °C was investigated using Macro TG. The obtained results showed that in rich O2 concentration, the particle size has little effect on char oxidation. However, the carbon conversion rate and the whole time of conversion decreases with an increase in particle size. There were some excepted cases contracting to this trend, mainly due to the difference in char samples' morphological structure at different sizes and operating temperatures in lean O2 concentration. The characteristics of biomass strongly affect the char oxidation rate. In all experiments, the conversion rate of char oxidation of the inside part of beefwood is higher than the bark's conversion rate. The oxidation rate of beech bark is higher than the one of the inside parts of the beech. There is no significant difference in char oxidation rate between the inside part of the beefwood and beech wood bark. However, all kinds of biomass in this study have a faster rate of oxidation than coal.