Abstract: To extend the oxygen enriched combustion technology in a large scale,it is necessary to study the emission characteristics of particulate matter in oxy-fuel combustion of pulverized coal. In this paper,the effect of H_2O(g) volume fraction(0,5%,10%,20%,30%) on the formation of ultrafine particulate matter(PM) under oxy-fuel combustion atmosphere was studied in a 1800 K drop tube furnace(DTF). Furthermore,the mass-based and number-based particle size distribution(PSD) were obtained and analyzed by a 14-stage electrical low pressure impactor(ELPI+). The results indicate that the mass-based and number-based PSD of ultrafine PMs remains almost unchanged in various H_2O(g) contents(0,5%,10%,20%,30%),while the peak of ultrafine particles fluctuates. The total number of ultrafine PM is determined by the number of ultrafine PM with the smallest particle size. The number fractions of the ultrafine PMs collected by the first impactor of ELPI + are all higher than 65% under all H_2O(g) contents. The total mass of ultrafine PM is determined by the mass of ultrafine with the largest particle size and the mass fractions of the ultrafine PMs collected by the seventh impactor of ELPI+are all higher than 94%. Low concentration of H_2O(g) can inhibit the formation of ultrafine particles,and the inhibition effect is the most significant when the volume fraction of H_2O(g) is 5%; high concentration of H_2O(g) can promote the formation of ultrafine particles. Because the gasification reaction of H_2O(g) with coal char produces a reducing atmosphere around the coal char particles,which promotes the reduction of minerals to elemental matter and further promotes the evaporation of minerals. On the other hand,the gasification reaction is endothermic reaction,which will reduce the combustion temperature of coal char particles. At the same time,the addition of H_2O(g) also leads to the increase of heat capacity of flue gas. The combustion temperature of coal char inhibits the evaporation of minerals in coal,resulting in the reduction of ultrafine particles,which is the result of the competition between the two kinds of interaction. In addition,the addition of H_2O(g) makes the average particle size of ultrafine particles increase,and the average particle size of ultrafine particles increases the most rapidly when 0-5% H_2O(g) is added.