The impacts of in-furnace kaolin addition on the formation and emission characteristics of PM2.5 from a 1000 MW coal-fired utility boiler equipped with electrostatic precipitators (ESPs) are investigated for the first time ever in this contribution. Detailed characterization of the chemical composition, micromorphology, melting characteristics of the fine PM, total fly ash, and/or bottom ash samples were carried out using the X-ray fluorescence probe, the field emission scanning electron microscope coupled with an energy dispersive X-ray detector, the ash fusion analyzer, and the dust specific resistivity analyzer. The results showed that the formation of fine PM was reduced when kaolin was added, and the mass concentrations of the particulate matter with the aerodynamic diameters of ≤0.3 and 2.5 μm (PM0.3 and PM2.5) were reduced by 55.97% and 5.48%, respectively. As expected, kaolin reacted with the volatile mineral vapors (e.g., Ca, Na) and inhibited their partitioning into ultrafine PM. It was interesting to find that the added kaolin modified the ash melting behavior, and promoted the capture of the ultrafine PM onto the coarse particles. What is more, the added kaolin reduced the specific resistivity of the fly ash and improved their capture efficiency in the ESPs. Finally, the above combined effects brought about the emission reductions of 41.27% and 36.72% for PM0.3 and PM2.5 after the ESPs. These results provided a direct confirmation on the feasibility of in-furnace kaolin addition on the PM reduction in the realistic combustion conditions.