This paper aims to clarify the effect of kaolin on the ash partitioning during combustion of a low-rank coal in O-2/ CO2 atmosphere. The coal sample was mixed mechanically with 10% kaolin and then combusted in a lab-scale drop tube furnace at 1400 degrees C in oxy-fuel atmosphere (28% O-2 balanced with CO2). The resulting ash was segregated as bottom ash, fine ash and ultrafine ash. The ash distribution as a function of the particle size of kaolin added was examined. The results indicated that the particle size range of kaolin with 106-150 mu m was the optimal for shifting the fine/ultrafine ash to the bottom ash, in which the total amount of fine and ultrafine ash was decreased by 18.1% compared with that of the raw coal combustion. The ultrafine ash with particle size less than 0.5 mu m generated from combustion of the coal mixed with kaolin (106-150 mu m) was decreased remarkably since alkali and alkaline earth metals (AAEM) including Na, K, Ca and Mg were captured by kaolin at high temperature. The formation of liquid layer on the surface of kaolin and coalescence of AAEM-rich particles were the main reason for shifting the fine/ultrafine ash to the bottom ash.