In this work, the formation of incipient ultrafine particulate matter (PM) from high-sodium lignite combustion, under both dry and wet oxy-fuel conditions, is intensively investigated using an optically accessible flat-flame burner. The ambient temperature was set at 1500 K, and measurements were performed along the zones with the coal residence time of 0-40 ms. We simultaneously introduced thermophoresis sampling, dilution sampling, and phase-selective laser-induced breakdown spectroscopy (PS-LIES) to characterize the morphologies, components, and particle size distributions (PSDs) of ultrafine PM as well as the behaviors of Na. The results indicate that incipient particles formed in CO2-O-2 ambience are more abundant in Na and Si. The substitution of CO, may accelerate the mass loss rate of coal in the early devolatilization stage, whereas the Na-based incipient particles form around a time of,10 ms that is similar to conventional N-2-O-2 ambience. In combination with the computational fluid dynamics (CFD), the PSD of ultrafine particles formed under the 70% CO2-30% O-2 condition is successfully predicted by a population balance model (PBM)-based model. Finally, the effect of H2O on the ultrafine PM formation at the start of oxy-fuel char combustion is examined. The apparent increment of Si-based ultrafine particles caused by H2O addition in oxy-fuel conditions is theoretically interpreted.