Fine particulate matter (PM2.5) was sampled at the furnace outlet of a 660 MW boiler using a two-stage sampling dilution system. The number and mass concentrations of PM2.5 were measured at different applied loads using an electrical lowpressure impactor to determine how boiler load affects PM2.5 production. Size-segregated elemental compositions of particles were measured by energy-dispersive X-ray spectroscopy. The results indicate that PM2.5 contains both ultrafine and central-mode particles. The number concentration depends on the ultrafine particles (PM0.38), and the mass concentration depends on the central-mode particles (PM0.38-2.5). The Na, S, and Ca concentrations decrease with increasing particle size, but Si, Al, and K concentrations in the central-mode particles are higher than in the ultrafine mode. The Fe and Mg concentrations are roughly independent of particle size. The trends of elemental behavior suggest that the central-mode particles are primarily formed from fine residual ash particles coated by heterogeneous condensation of vaporized minerals. With the boiler load increasing, the number and mass concentrations of PM2.5, PM0.38, and PM0.38-2.5 increased, which was mainly due to the higher furnace temperature. The average size of PM0.38 tended to increase with increasing load, but the PM0.38-2.5 average size decreased: this difference is attributed to their different mechanisms of formation.