To characterize the primary PM2.5 emission from coal-fired power plants in China, and to quantitatively evaluate the effects of flue gas denitrification and desulfurization on PM2.5 emission, a pulverized coal fired (PC) power plant and a circulating fluidized bed (CFB) plant were selected for measuring the mass concentration and water-soluble ion composition of PM2.5 in flue gas. The results showed that the mass concentration of PM2.5 generated from the CFB was much higher than that from the PC, while the mass concentrations of PM2.5 emitted from these two plants were very similar, because the CFB was equipped with an electrostatic-bag precipitator (EBP) with higher PM2.5 removal efficiency than the common electrostatic precipitator (ESP). Although the total concentration of water-soluble ions in PM2.5 generated from the PC was lower than that from the CFB, the total concentration of water-soluble ions in PM2.5 emitted from the PC was much higher than that from the CFB, which implied that PM2.5 emission from the PC was greatly affected by the flue gas treatment installations. For example, the flue gas denitrification system produced H2SO4 mist, part of which reacted with the excessive NH3 in the flue gas to form NH4HSO4 in PM2.5 and to increase the acidity of PM2.5. In addition, the escaping of desulfurization solution during the flue gas desulfurization process could also introduce NH4+ and SO2- into PM2.5. Therefore, although the main water-soluble ions in PM2.5 generated from both of the plants were Ca2+ and SO(4)2-, the major cation was changed to NH4+ when emitted from PC.