The present paper was addressed toward the impact of O-2/CO2 combustion on mineral transformation and fine ash formation. A high-aluminum coal was burned with an O-2/CO2 mixture in a drop tube furnace. The collected ash samples were characterized in details to study the ash formation behaviors, and the comparison was made between O-2/CO2 combustion and air combustion. It was found that boehmite transformed to theta-Al2O3 and then to alpha-Al2O3 and the extent of the transformation depended upon the residence time and more significantly upon the particle combustion temperature. In comparison to air combustion, O-2/CO2 combustion did not affect the species of mineral phases formed in the ashes of the coal studied but did affect the relative amounts of the phases. O-2/CO2 combustion had an impact on the coal particle combustion temperature and consequently on the ash mineral composition. O-2/CO2 combustion decreased the yields of the fine ash particles in both the submicrometer fume region and fine fragmentation region as compared to air combustion with the same O-2 concentration because of the decrease in the particle combustion temperature, while an increasing O-2 concentration enhanced the formation of both region particles. The mode size of submicrometer particles formed in O-2/CO2 combustion was found shifting to a smaller size when compared to that in air combustion.