Coal/biomass co-combustion experiments were carried out in a laboratory-scaled drop tube furnace to understand the emission characteristics of particulate matter with an aerodynamic diameter of less than 10 mu m (PM10). The furnace was electrically heated, and the temperature was maintained at 1423 K. Two different atmospheres, N-2/O-2 = 4:1 and 1:1, were conducted. The coal/biomass ratio was kept at 75:25% (wt %), and four types of coal/biomass blends were selected as fuels in these experiments. The fuel feeding rate was adjusted to about 0.3 g/min. The results show that the particle-size distribution of PM10 from the co-combustion of coal and biomass is bimodal, with one peak at about 4.3 mu m and the other at about 0.1 mu m. With the increase of the oxygen ratio, the total concentration of PM10 increases significantly and the percentage of PM1.0 (particle size below 1.0 mu m) in PM10 decreases greatly, while that of PM1.0+ (particle size between similar to 1.0-10 mu m) increases. In addition, the majority of alkali, sulfur, and chlorine are enriched in PM1.0. With the increase of the oxygen concentration, the ratio of S/Cl increases and a higher sulfating extent is found in PM1.0. Meanwhile, the alkali content in PM1.0 decreases, while there is a significant increase in PM1.0+. In addition to the combination with chlorine or sulfur, part of alkalis in coarse particles is found to form other alkali salts.