Abstract: The influence of kaolinite additive on the emission characteristics of PM_2.5 (particulates with aerodynamic diameter less than 2.5μm) was studied with a tube furnace. The combustion tests were carried out at 1123K under O₂/CO₂ atmosphere. The PM_2.5 generated from coal combustion was collected and analyzed with an Electrical Low Pressure Impactor (ELPI). The results indicate that kaolinite is an important factor for the formation of PM_2.5 during coal combustion under O₂/CO₂ atmosphere. The number and mass concentrations of PM₁ diminish, but those of PM_1~2.5 enhance slightly after kaolinite is added. The size distributions of PM_2.5are similar, which display two peaks around 0.2μm and 2.0μm, respectively. With increasing the weight ratio of kaolinite, the concentrations of S, Pb, Cu, Na and K decrease. The submicron-size ash particles smaller than 0.317μm are formed via nucleation of vaporized ash components. The supermicron-size ash particles are formed by coagulation and coalescence of the submicron-size ash, and fragmentation and coalescence of mineral matter.