Hospital solid waste incinerator (HSWI) fly ash is an extremely complex mixture because it contains carbon constituents, toxic heavy metals, and polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). We investigated the distribution of carbon constituents, PCDD/Fs, and heavy metals in the five different particle size fractions (+106 µm, –106 + 75 µm, –75 + 38 µm, –38 + 25 µm, and –25 µm) of HSWI fly ash. The loss on ignition (LOI) in the particle size fraction of –25 µm was higher than that in the particle size fractions of other sizes. Moreover, 82.63% of the fly ash particles were in the size range of –75 + 25 µm, and this particle size range was suitable for direct flotation treatment. Because powder-activated carbon adsorbed a higher amount of low-chlorinated PCDD/Fs in the gaseous phase, the concentration of each PCDD/F congener, particularly of low-chlorinated PCDD/Fs, generally increased with a decrease in the particle size. The total toxic equivalent (TEQ) of the fraction of –75 µm did not satisfy the landfill site standard (3 ng-TEQ g–1). The highest concentration of Pb, Zn, and Cd was observed in the fraction of –38 + 25 µm rather than in the fraction of –25 µm. The exchangeable speciation of Zn, Pb, and Cd was relatively high in the particle size fraction of –75 µm. The carbonate speciation of Pb and Zn was the highest in the particle size fraction of –38 + 25 µm. The leaching concentrations of Pb and Cd in all particle size fractions exceeded landfill allowable thresholds.