Abstract: The effect of limestone addition on the emission characteristics of PM_2.5 (particulates with aerodynamic diameter less than 2.5 μm) was studied in a fluidized bed combustor. The combustion tests were carried out at 1 123 K 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 limestone has a great effect on the formation of PM_2.5 during coal combustion under O₂/CO₂ atmosphere. The concentration of PM_1.0 is diminished; but the concentration of PM_1.0~2.5 is enhanced slightly with the addition of limestone. The mass size distribution of PM_2.5 is similar, which displays two peaks around 0.2 and 2.0 μm, respectively. As the molar ratio of Ca/S increases, the concentrations of Si, Na, K, S, and Cu decrease. As the particle size decreases, the concentrations of S, Cu, K, and Na increase, but the concentrations of Si decrease.